[XFS] Remove unused HAVE_SPLICE macro.

HAVE_SPLICE was part of the infrastructure for building 2.4 and 2.6
kernels out of the same tree. Now we don't build 2.4 kernels this

SGI-PV: 971046
SGI-Modid: xfs-linux-melb:xfs-kern:30878a

Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>