Revert "reland attempt 3: Adds WindowService-ChromeOS fieldtrial"

This reverts commit d13c288b.

Reason for revert: Two serious issues have come up: 822917 and 822577.

Original change's description:
> reland attempt 3: Adds WindowService-ChromeOS fieldtrial
> 
> I'm not sure on the name here. I went with WindowService-ChromeOS as
> at some point we will launch the WindowService on other platforms, but
> that won't be anytime soon and is likely to be very different. So, I'm
> thinking later on we have the field trial WindowService-Desktop.
> 
> BUG=814898
> TEST=none
> TBR=rkaplow@chromium.org
> 
> Change-Id: I555c9cea0f59c28b2e6124196f362ac3231104fc
> Reviewed-on: https://chromium-review.googlesource.com/962877
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#543156}

TBR=sky@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 814898
Change-Id: I2290fd6ee56d985117b772f4554eac6e2e658b99
Reviewed-on: https://chromium-review.googlesource.com/966694Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543886}

testing/variations/fieldtrial_testing_config.json

@@ -4337,21 +4337,6 @@
             ]
         }
     ],
-    "WindowService-ChromeOS": [
-        {
-            "platforms": [
-                "chromeos"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "Mus"
-                    ]
-                }
-            ]
-        }
-    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [