KVM: remove useless write to vcpu->hv_clock.tsc_timestamp

After the previous patch from Marcelo, the comment before this write
became obsolete.  In fact, the write is unnecessary.  The calls to
kvm_write_tsc ultimately result in a master clock update as soon as
all TSCs agree and the master clock is re-enabled.  This master
clock update will rewrite tsc_timestamp.

So, together with the comment, delete the dead write too.
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -1278,8 +1278,6 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	kvm->arch.last_tsc_write = data;
 	kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
 
-	/* Reset of TSC must disable overshoot protection below */
-	vcpu->arch.hv_clock.tsc_timestamp = 0;
 	vcpu->arch.last_guest_tsc = data;
 
 	/* Keep track of which generation this VCPU has synchronized to */