x86: remove the write-only timer_uses_ioapic_pin_0

This patch removes the write-only timer_uses_ioapic_pin_0
(gsi can't be <= 15 in the line of it's fake usage in mpparse_32.c).

Spotted by the GNU C compiler.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>