ipmi: remove ->write_proc code

IPMI code theoretically allows ->write_proc users, but nobody uses this thus
far.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>