m68k: Remove never used asm/shm.h

m68k's asm/shm.h header has been part of the tree ever since m68k
support got added in v1.3.94. (It started as /include/asm-m68k/shm.h and
moved to its current location a few years ago.) It seems it was never
used: no file ever included it and nothing used the macros it defines.

(Actually, from v2.5.46 until v2.6.29-rc3 it was included by m68knommu's
asm/shm.h. But that header was just a very thin wrapper for this header
and was itself unused too.)

This header can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>