Stop the hover popup from appearing when it shouldn't

Resolves #1884 

This bug occurs because the callback is not executed (so the timeout is not removed) when `fadeElementTo` returns early (when fading is not necessary).

I think this change produces logical behavior, as we expect `RESUtils.fadeElementTo` to change the opacity and then execute the callback when it is completed (so it is still technically completed even if it didn't have to do anything).

User hover, subreddit hover, parents, and notifications seem to still work, also.