Btrfs: remove no longer used function to run delayed refs asynchronously

It used to be called from only two places (truncate path and releasing a
transaction handle), but commits 28bad212 ("btrfs: fix truncate
throttling") and db2462a6 ("btrfs: don't run delayed refs in the end
transaction logic") removed their calls to this function, so it's not used
anymore. Just remove it and all its helpers.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>