qemu: remove obsolete qemuMonitorMigrateToCommand

qemu: remove obsolete qemuMonitorMigrateToCommand

The last use of qemuMonitorMigrateToCommand was removed years back in

  commit 2e90c9daf9eabe15d826a48c06607acb9d18fda6
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Fri Nov 6 16:50:26 2015 +0000

    qemu: assume support for all migration protocols except rdma

Prior to that commit, 'exec:' to used to replicate the 'unix:' protocol
by spawning 'nc'.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>