Remove WebFontsIntervention flag from fieldtrial testing config

The feature was already shipped, and there is no relevant trial
on trunk source code.

Note: This configuration looks wrong. Since the study used FeatureList
API, it should have an enable_features field to enable it correctly.

BUG=n/a

Review-Url: https://codereview.chromium.org/2046463002
Cr-Commit-Position: refs/heads/master@{#398136}

testing/variations/fieldtrial_testing_config_android.json

@@ -496,11 +496,6 @@
             }
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [

testing/variations/fieldtrial_testing_config_chromeos.json

@@ -272,11 +272,6 @@
             }
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [

testing/variations/fieldtrial_testing_config_linux.json

@@ -327,11 +327,6 @@
             }
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [

testing/variations/fieldtrial_testing_config_mac.json

@@ -372,11 +372,6 @@
             "group_name": "Enabled"
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [

testing/variations/fieldtrial_testing_config_win.json

@@ -428,11 +428,6 @@
             }
         }
     ],
-    "WebFontsIntervention": [
-        {
-            "group_name": "Enabled"
-        }
-    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [