xfs: Remove duplicate assert statement in xfs_bmap_btalloc()

The check for verifying if the allocated extent is from an AG whose
index is greater than or equal to that of tp->t_firstblock is already
done a couple of statements earlier in the same function. Hence this
commit removes the redundant assert statement.

Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>