Remove WebMimeRegistry::preferredExtensionForMIMEType stub.

The Chromium implementations of this have been removed, so this can now
safely go away.

BUG=268948

Review URL: https://chromiumcodereview.appspot.com/22796007

git-svn-id: svn://svn.chromium.org/blink/trunk@156178 bbb929c8-8fbe-4397-9dbb-9b2b20218538