[XFS] Remove the xfs_refcache

Remove the xfs_refcache, it was only needed while we were still
building for 2.4 kernels.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30472a

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