release/deploy: clean up old agent extensions on DZDs after deploying

[snormore]

After deploying the config and telemetry agents, old extensions should be deleted.

Use something like this to get list of extensions that aren't installed:

chi-dn-dzd1#show extensions | json
{
  "extensions": {
      "doublezero-device-telemetry-agent_0.0.2git20250711140449.ced957a_linux_amd64.rpm": {
          "version": "0.0.2~git20250711140449.ced957a",
          "release": "1",
          "presence": "present",
          "status": "notInstalled",
          "boot": false,
          "numPackages": 1,
          "error": false,
          "vendor": "doublezero",
          "summary": "DoubleZero agent telemetry",
          "installedSize": 0,
          "packages": {},
          "description": "DoubleZero agent telemetry",
          "affectedAgents": [],
          "agentsToRestart": []
   }
},

And remove them

delete flash:doublezero-agent_X.Y.Z_linux_amd64.rpm
delete extension:doublezero-agent_X.Y.Z_linux_amd64.rpm