Remove WebFontsIntervention flag from fieldtrial testing config

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 354 matching lines @@
         }
     ],
     "ViewsSimplifiedFullscreenUI": [
         {
             "enable_features": [
                 "ViewsSimplifiedFullscreenUI"
             ],
             "group_name": "Enabled"
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [
                 "WebRTC-EnableWebRtcEcdsa"
             ],
             "group_name": "Enabled"
         }
     ],
     "WebRTC-H264WithOpenH264FFmpeg": [
         {
@@ skipping 15 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }