Do not preserve metal-agent extension when installing Talos to disk

[utkuozdemir]

Problem:

• Provider boots Talos into agent mode
• User accepts machine on Omni
• Omni creates a MachineStatus with the list of extensions and schematic ID in it, including all firmware extensions and metal-agent extension
• User adds the machine to a cluster
• Provider PXE boots Talos
• Before config is pushed to install Talos to disk, Omni
  • either misses to update set of extensions in MachineStatus
  • or something else happens (?)
• Talos gets installed to the disk with the metal-agent
• It is a broken Talos installation

We have a temporary fix for it, but need to find the right solution.