xfs: remove qcore from incore dquots

Now that we've stopped using qcore entirely, drop it from the incore
dquot.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>