[XFS] kill XFS_INOBT_IS_FREE_DISK

This macro is unused an all other acros in this family operate on native
types, so we most likely won't grow a user either.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29846a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>