[Signin] Remove redundant testing config for MirrorMultilogin

The feature has been rolled out and the flag removed.

Bug: None
Change-Id: I0579819f0d079cb24363fcad5326349be098bc90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489633Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820988}

testing/variations/fieldtrial_testing_config.json

@@ -4336,22 +4336,6 @@
             ]
         }
     ],
-    "MirrorMultilogin": [
-        {
-            "platforms": [
-                "android",
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "UseMultiloginEndpoint"
-                    ]
-                }
-            ]
-        }
-    ],
     "MixedFormWarnings": [
         {
             "platforms": [