arm/arm64: KVM: Clean up useless code in kvm_timer_enable

1) Since commit:41a54482 changed timer enabled variable to per-vcpu,
   the correlative comment in kvm_timer_enable is useless now.

2) After the kvm module init successfully, the timecounter is always
   non-null, so we can remove the checking of timercounter.
Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

virt/kvm/arm/arch_timer.c

@@ -498,17 +498,7 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
 	if (ret)
 		return ret;
 
-
-	/*
-	 * There is a potential race here between VCPUs starting for the first
-	 * time, which may be enabling the timer multiple times.  That doesn't
-	 * hurt though, because we're just setting a variable to the same
-	 * variable that it already was.  The important thing is that all
-	 * VCPUs have the enabled variable set, before entering the guest, if
-	 * the arch timers are enabled.
-	 */
-	if (timecounter)
-		timer->enabled = 1;
+	timer->enabled = 1;
 
 	return 0;
 }