Revert "Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode."

This reverts commit 9d6a01ba.

Reason for revert: Breaking GPU tests on GPU FYI waterfall and on ANGLE's CQ.
Bug: 1014659

Original change's description:
> Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode.
> 
> This experiment enables:
> - Use THREAD_MODE_BACKGROUND_BEGIN/END to set the priority of
>   background threads. This affects memory and IO priority in addition
>   to CPU priority.
> - Increase thread priority to "normal" in scopes where background
>   threads load modules.
> 
> The expectation is that this will improve responsiveness of Chrome
> (input delay, first contentful paint), without regressing jank.
> 
> We are adding the testing config in preparation for enabling this
> via Finch in M79.
> 
> Bug: 973868
> Change-Id: I467cae12503e3ad724b665f3aebdaeb94e1325c9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835023
> Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Auto-Submit: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#705960}

TBR=fdoray@chromium.org,rkaplow@chromium.org

Change-Id: I509cbe39c2d4f91859f5e8614d7d8f8d819d3a7d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 973868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862858Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706145}

testing/variations/fieldtrial_testing_config.json

@@ -1434,22 +1434,6 @@
             ]
         }
     ],
-    "BoostThreadPriorityOnLibraryLoadingAndBackgroundMode": [
-        {
-            "platforms": [
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "BoostPriorityAndBackgroundThreadMode",
-                    "enable_features": [
-                        "BoostThreadPriorityOnLibraryLoading",
-                        "WindowsThreadModeBackground"
-                    ]
-                }
-            ]
-        }
-    ],
     "BrowserBlacklist": [
         {
             "platforms": [