Remove the obsolete MinidumpDirectoryObserver experiment.

testing/variations/fieldtrial_testing_config_android.json

 {
     "AllBookmarks": [
         {
             "disable_features": [
                 "AllBookmarks"
             ],
             "group_name": "Disabled"
         }
     ],
     "AndroidSessionNotifications": [
@@ skipping 180 matching lines @@
         }
     ],
     "MainFrameBeforeActivation": [
         {
             "enable_features": [
                 "MainFrameBeforeActivation"
             ],
             "group_name": "Enabled"
         }
     ],
-    "MinidumpDirectoryObserver": [
-        {
-            "group_name": "ObserverExperimentGroup",
-            "params": {
-                "Enabled": "true"
-            }
-        }
-    ],
     "ModerateBindingOnBackgroundTabCreation": [
         {
             "group_name": "Enabled"
         }
     ],
     "MojoChannel": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 374 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }