Remove stale fieldtrial_testing_configs.

This CL removes all studies in fieldtrial_testing_config.json where
neither the study name nor any of enabled or disabled feature names
appear as a string literal elsewhere in the codebase.

This CL has been generated semi-automatically:
1. Download cleanup.py attached to https://crbug.com/1053702#c3
2. Generate cleaned-up list of studies:
   $ python3 ~/Downloads/cleanup.py \
         testing/variations/fieldtrial_testing_config.json \
         ~/scratch/out.txt
3. Copy the output:
   $ cp ~/scratch/out.txt \
         testing/variations/fieldtrial_testing_config.json
4. Format the output:
   $ python testing/variations/PRESUBMIT.py \
         testing/variations/fieldtrial_testing_config.json

Bug: 1053702
Change-Id: If7a17f06786bfa8dbda312401d291bf120251aff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363717
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799664}

testing/variations/fieldtrial_testing_config.json

@@ -879,24 +879,6 @@
             ]
         }
     ],
-    "BrowserScheduler": [
-        {
-            "platforms": [
-                "windows",
-                "mac",
-                "chromeos",
-                "linux",
-                "android",
-                "android_weblayer",
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "DetachTimeFg16x"
-                }
-            ]
-        }
-    ],
     "BrowserVerifiedUserActivationKeyboardTrigger": [
         {
             "platforms": [
@@ -4027,27 +4009,6 @@
             ]
         }
     ],
-    "MaxOverlapBoundsForFixed": [
-        {
-            "platforms": [
-                "android",
-                "android_weblayer",
-                "android_webview",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "MaxOverlapBoundsForFixed"
-                    ]
-                }
-            ]
-        }
-    ],
     "MediaApp": [
         {
             "platforms": [
@@ -4169,25 +4130,6 @@
             ]
         }
     ],
-    "MergeBlockingNonBlockingPools": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Disabled",
-                    "disable_features": [
-                        "MergeBlockingNonBlockingPools"
-                    ]
-                }
-            ]
-        }
-    ],
     "MirrorMultilogin": [
         {
             "platforms": [
@@ -5174,24 +5116,6 @@
             ]
         }
     ],
-    "PolicyAtomicGroup": [
-        {
-            "platforms": [
-                "windows",
-                "chromeos",
-                "mac",
-                "linux"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "PolicyAtomicGroup"
-                    ]
-                }
-            ]
-        }
-    ],
     "PreconnectOnDidFinishNavigation": [
         {
             "platforms": [
@@ -7027,22 +6951,6 @@
             ]
         }
     ],
-    "TranslateSubFramesStudy": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "TranslateSubFrames"
-                }
-            ]
-        }
-    ],
     "TriggeredResetFieldTrial": [
         {
             "platforms": [
@@ -7632,21 +7540,6 @@
             ]
         }
     ],
-    "WebRTC-Audio-NewOpusPacketLossRateOptimization": [
-        {
-            "platforms": [
-                "windows",
-                "mac",
-                "chromeos",
-                "linux"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled-0-20-1"
-                }
-            ]
-        }
-    ],
     "WebRTC-Audio-SendSideBwe": [
         {
             "platforms": [