uapi: libc-compat: remove ipx leftovers

The uAPI headers for IPX were deleted 3 years ago in
commit 6c9b4084 ("net: Remove net/ipx.h and uapi/linux/ipx.h header files")
Delete the leftover defines from libc-compat.h

Link: https://patch.msgid.link/20240911002142.1508694-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>