xfs: remove a __xfs_bunmapi call from reflink

This raw call isn't necessary since we can always remove a full delalloc
extent.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>