Remove expired cache pixbuf, don't just ignore it

Previous fix wasn't quite there.  Better to evict expired pixbuf cache entries immediately instead of leaving them cluttering up the cache until they get pushed out by age.  With this change, the whole cache expiry check on reset is no longer necessary.  The artwork plugin will check against the reset time when get_album_art() is called and it will update the disk cache file, then the pixbuf cache will be checked against disk and updated if necessary.