Media Controls: Remove fieldtrial for modern media controls

This CL removes the entry in fieldtrial_testing_config.json for the
modern media controls, since the legacy controls no longer exist.

Bug: 884215
Change-Id: Ibc21ac0c61288581e2625821ca730697c834981c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1673752Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671785}

testing/variations/fieldtrial_testing_config.json

@@ -3051,25 +3051,6 @@
             ]
         }
     ],
-    "ModernMediaControls": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "UseModernMediaControls"
-                    ]
-                }
-            ]
-        }
-    ],
     "MojoChannel": [
         {
             "platforms": [