xfs: remove useless definitions in xfs_linux.h

Remove current_pid(), current_test_flags() and
current_clear_flags_nested(), because they are useless.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>