xfs: remove bogus assertion when online repair isn't enabled

We don't need to assert on !REPAIR in the stub version of
xrep_calc_ag_resblks that is called when online repair hasn't been
compiled into the kernel because none of the repair code will ever run.

Reported-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>