xfs: remove unnecessary assertion from xfs_qm_vop_create_dqattach

The check XFS_IS_QUOTA_RUNNING() has been done when enter the
xfs_qm_vop_create_dqattach() function, it will return directly
if the result is false, so the followed XFS_IS_QUOTA_RUNNING()
assertion is unnecessary. If we truly care about this, the check
also can be added to the condition of next if statements.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>