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

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 99 matching lines @@
     "IntelligentSessionRestore": [
         {
             "group_name": "Enabled"
         }
     ],
     "LocalNTPSuggestionsService": [
         {
             "group_name": "Enabled"
         }
     ],
-    "MaterialDesignDownloads": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "NetworkQualityEstimator": [
         {
             "group_name": "Enabled",
             "params": {
                 "2G.DefaultMedianKbps": "81",
                 "2G.DefaultMedianRTTMsec": "1337",
                 "3G.DefaultMedianKbps": "658",
                 "3G.DefaultMedianRTTMsec": "297",
                 "4G.DefaultMedianKbps": "1756",
                 "4G.DefaultMedianRTTMsec": "159",
@@ skipping 172 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }