x86: I/O APIC: remove redundant LVT0 masking

The LINT0 line of the local APIC is masked in the LVT0 entry in
check_timer() before this function is ever called.  Removed the
redundant unmasking for better control.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>