xfs: Remove dead function prototype xfs_sync_inode_grab()

Remove dead function prototype xfs_sync_inode_grab()
from xfs_icache.h.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>