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: I100d97a68aabeba2f7c182cc9ddc478314d71dde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379730
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803456}

testing/variations/fieldtrial_testing_config.json

@@ -2261,22 +2261,6 @@
             ]
         }
     ],
-    "EduCoexistence": [
-        {
-            "platforms": [
-                "chromeos"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "EduCoexistence",
-                        "EduCoexistenceConsentLog"
-                    ]
-                }
-            ]
-        }
-    ],
     "EnableAutofillPasswordReauthIOS": [
         {
             "platforms": [
@@ -7593,27 +7577,6 @@
             ]
         }
     ],
-    "WebPaymentsPerMethodCanMakePaymentQuota": [
-        {
-            "platforms": [
-                "android",
-                "android_weblayer",
-                "chromeos",
-                "ios",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "WebPaymentsPerMethodCanMakePaymentQuota"
-                    ]
-                }
-            ]
-        }
-    ],
     "WebProtectConnectors": [
         {
             "platforms": [