KVM: LAPIC: Drop pending LAPIC timer injection when canceling the timer

The timer is disarmed when switching between TSC deadline and other modes;
however, the pending timer is still in-flight, so let's accurately remove
any traces of the previous mode.

Fixes: 44275932

 ("KVM: x86: thoroughly disarm LAPIC timer around TSC deadline switch")
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>