Remove reference to enum that never existed

The virDomainSendProcessSignal method says the flags values
come from virDomainProcessSignalFlag, but this enum has
never existed. No flags are needed for this method.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/libvirt-domain.c

@@ -6866,7 +6866,7 @@ virDomainSendKey(virDomainPtr domain,
  * @domain: pointer to domain object
  * @pid_value: a positive integer process ID, or negative integer process group ID
  * @signum: a signal from the virDomainProcessSignal enum
- * @flags: one of the virDomainProcessSignalFlag values
+ * @flags: currently unused, pass 0
  *
  * Send a signal to the designated process in the guest
  *