Bug 1685917 - remove layout.css.file-selector-button.enabled pref following bug 1685917. r=darktrojan

It's on by default since bug 1662478 (Firefox 82)

Corresponding m-c commit: https://hg.mozilla.org/mozilla-central/rev/b0e30bc70520

mail/test/static/browser_parsable_css.js

@@ -94,22 +94,13 @@ if (
   });
 }
 
-if (Services.prefs.getBoolPref("layout.css.file-selector-button.enabled")) {
-  // System colors reserved to UA / chrome sheets
-  whitelist.push({
-    sourceName: /(?:res|gre-resources)\/forms\.css$/i,
-    errorMessage: /Expected color but found \u2018-moz.*/i,
-    platforms: ["linux"],
-    isFromDevTools: false,
-  });
-} else {
-  // Reserved to UA sheets, behind a pref for content.
-  whitelist.push({
-    sourceName: /(?:res|gre-resources)\/forms\.css$/i,
-    errorMessage: /Unknown pseudo-.*file-selector-button/i,
-    isFromDevTools: false,
-  });
-}
+// System colors reserved to UA / chrome sheets
+whitelist.push({
+  sourceName: /(?:res|gre-resources)\/forms\.css$/i,
+  errorMessage: /Expected color but found \u2018-moz.*/i,
+  platforms: ["linux"],
+  isFromDevTools: false,
+});
 
 if (!Services.prefs.getBoolPref("layout.css.scroll-anchoring.enabled")) {
   whitelist.push({