init.h: remove long-dead __setup_null_param() macro

This macro appears to have been unused for ages, and there are no
invocations of it anywhere in the source tree.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>