Revert "Adds WindowService-ChromeOS fieldtrial"

This reverts commit b4603176.

Reason for revert: Reverting as one of the fixs for an interactive_ui_test failure got reverted.

Original change's description:
> 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
> 
> Change-Id: I5bf45c2a463de0fefaeb06939429977e36c3681e
> Reviewed-on: https://chromium-review.googlesource.com/932821
> Commit-Queue: Scott Violet <sky@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542680}

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

Change-Id: Id63c00a90c6fd05ff74f0ffaa6ed738a98bd74e3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 814898
Reviewed-on: https://chromium-review.googlesource.com/959763Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542713}

testing/variations/fieldtrial_testing_config.json

@@ -4290,21 +4290,6 @@
             ]
         }
     ],
-    "WindowService-ChromeOS": [
-        {
-            "platforms": [
-                "chromeos"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "Mus"
-                    ]
-                }
-            ]
-        }
-    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [