btrfs: Remove unused locking functions

Those were split out of btrfs_clear_lock_blocking_rw by
aa12c027 ("btrfs: split btrfs_clear_lock_blocking_rw to read and write helpers")
however at that time this function was unused due to commit
52398340 ("Btrfs: kill btrfs_clear_path_blocking"). Put the final
nail in the coffin of those 2 functions.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>