Remove MixedFormWarnings from testing config

Features were enabled by default in crrev.com/c/2583141 and are no
longer required to be in fieldtrial_testing_config.json

Bug: 1105210
Change-Id: Ie984ea0e17d151cf9105ab85ba11e6a4c4762457
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586771
Commit-Queue: Carlos IL <carlosil@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Auto-Submit: Carlos IL <carlosil@chromium.org>
Reviewed-by: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836283}

testing/variations/fieldtrial_testing_config.json

@@ -4568,26 +4568,6 @@
             ]
         }
     ],
-    "MixedFormWarnings": [
-        {
-            "platforms": [
-                "windows",
-                "android",
-                "chromeos",
-                "linux",
-                "mac"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "AutofillPreventMixedFormsFilling",
-                        "InsecureFormSubmissionInterstitial"
-                    ]
-                }
-            ]
-        }
-    ],
     "MobileIdentityConsistency": [
         {
             "platforms": [