Remove unused _handlePrefsTap function from gr-diff-preferences

_handlePrefsTap was copied to this file in Id7396147e73354122ea3825bde2c324b5daa1d26
but was never called by anything.

Change-Id: I76342e837b820f20df2d4a7dce586d0ef0c17241

polygerrit-ui/app/elements/diff/gr-diff-preferences/gr-diff-preferences.js

@@ -120,11 +120,6 @@
       this.$.prefsOverlay.close();
     },
 
-    _handlePrefsTap(e) {
-      e.preventDefault();
-      this._openPrefs();
-    },
-
     open() {
       this.$.prefsOverlay.open().then(() => {
         const focusStops = this.getFocusStops();