Remove old downloads UI; Material Design version is now the default.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 79 matching lines @@
     "LocalNTPSuggestionsService": [
         {
             "group_name": "Enabled"
         }
     ],
     "MacMemoryMechanism": [
         {
             "group_name": "Mach"
         }
     ],
-    "MaterialDesignDownloads": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "PageRevisitInstrumentation": [
         {
             "group_name": "Enabled"
         }
     ],
     "PasswordBranding": [
         {
             "group_name": "SmartLockBrandingSavePromptOnly"
         }
     ],
@@ skipping 118 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }