Revert "Reland "Add field trial testing config for...

Revert "Reland "Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode.""

This reverts commit d97ee8ce.

Reason for revert: Still has the same failure mode.

Bug: 1014659

Original change's description:
> Reland "Add field trial testing config for BoostThreadPriorityOnLibraryLoadingAndBackgroundMode."
> 
> This is a reland of 9d6a01ba
> We now handle negative values returned from ::GetThreadPriority().
> https://chromium-review.googlesource.com/c/chromium/src/+/1829570
> 
> 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=rkaplow@chromium.org
> 
> Bug: 973868
> Change-Id: I392cfb2f92e2ac0fa888fa1e323557ab35055b48
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872312
> Reviewed-by: François Doray <fdoray@chromium.org>
> Commit-Queue: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#707822}

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

Change-Id: I814a086aea8caacacd09d4fb507c433b5fe7ebdc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 973868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872017Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708033}

testing/variations/fieldtrial_testing_config.json

@@ -1419,22 +1419,6 @@
             ]
         }
     ],
-    "BoostThreadPriorityOnLibraryLoadingAndBackgroundMode": [
-        {
-            "platforms": [
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "BoostPriorityAndBackgroundThreadMode",
-                    "enable_features": [
-                        "BoostThreadPriorityOnLibraryLoading",
-                        "WindowsThreadModeBackground"
-                    ]
-                }
-            ]
-        }
-    ],
     "BrowserBlacklist": [
         {
             "platforms": [