Delete metal3MachineTemplates on mgmtCluster after pivot

Close #1840 (closed)

In order for the metal3machineTemplates to contain the correct OS image URL, it is necessary to delete the pivoted m3mTs on the mgmtCluster.
Indeed, these resources will be recreated with values corresponding to the mgmtcluster via the HelmRelease cluster. And since the DriftDetection is not yet enabled, this is a quick and easy solution to implement before the release.
To avoid the cluster remaining without the machinestemplates resources for too long, we realized this deletion just after the first reconciliation of the cluster helmrelease. Then we force the reconciliation of the helmrelease again to allow the machinestemplates resources to be recreated.\

(The solution of removing the OwnerReference from the bootstrap m3mt to prevent it from being pivoted is too uncertain, as the controller adds the OwnerReference back each time after it is removed. It would be necessary to add a new step to pause the cluster during the bootstrap phase.)