isdn/sc: remove unused REQUEST_IRQ and unnecessary header file

REQUEST_IRQ is never used, so delete it. In the process get rid of the
macro FREE_IRQ which makes the code unnecessarily difficult to read.

Signed-off-by: Fernando Luis Vázquez Cao <fernando@oss.ntt.co.jp>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>