m68k: Drop redundant generic-y += hardirq.h

The cleanup in commit 630f289b ("asm-generic: make more
kernel-space headers mandatory") did not take into account the recently
added line for hardirq.h in commit acc45648 ("m68k: Switch to
asm-generic/hardirq.h"), leading to the following message during the
build:

    scripts/Makefile.asm-generic:25: redundant generic-y found in arch/m68k/include/asm/Kbuild: hardirq.h

Fix this by dropping the now redundant line.

Fixes: 630f289b ("asm-generic: make more kernel-space headers mandatory")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>