Get rid of final remnants of include/asm-$(ARCH)

This renames include/asm-h8300/timer.h into arch/h8300/include/asm: it
was left over just because that file had been created in the -mm tree
before the whole h8300 header subdirectory had been moved, and then got
merged in the old location afterwards.

(See commits e0b0f9e4: "h8300: update
timer handler - new files" and 758db3f2:
"[h8300] move include/asm-h8300 to arch/h8300/include/asm" for details).

This also removes a left-over .gitignore file in include/asm-arm that
became stale when the ARM header files were moved (which happened in
multiple commits, just see "git log -- include/asm-arm" for details).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>