btrfs: remove btrfs_lru_cache_is_full() inline function

It's not used anywhere at the moment, but it was used in earlier version
of a patch that removed its use in the second version. So just remove
btrfs_lru_cache_is_full().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>