KVM: nVMX: fix comments of handle_vmon()

"VMXON pointer" is saved in vmx->nested.vmxon_ptr since
commit 3573e22c

 ("KVM: nVMX: additional checks on
vmxon region"). Also, handle_vmptrld() & handle_vmclear()
now have logic to check the VMCS pointer against the VMXON
pointer.

So just remove the obsolete comments of handle_vmon().

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Message-Id: <20210908171731.18885-1-yu.c.zhang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>