xfs: remove redundant checks from xfs_da_read_buf

All of the verification checks of magic numbers are now done by
verifiers, so ther eis no need to check them again once the buffer
has been successfully read. If the magic number is bad, it won't
even get to that code to verify it so it really serves no purpose at
all anymore. Remove it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>