um: page_offset.h is never used

... and neither is the only define in it
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/include/asm/page_offset.h

-#define PAGE_OFFSET_RAW (uml_physmem)