Remove the `napalm_driver` and `napalm_args` fields from Platform

[jeremystretch]

Proposed Changes

Remove the napalm_driver and napalm_args fields from the Platform model.

Justification

NAPALM integration was moved to a plugin in NetBox v3.5 under #10520. These fields were retained to ease migration to the plugin, but are no longer needed once the plugin has been installed.

[PieterL75]

@jeremystretch Can we keep the 'driver' field ?
My automation uses napalm, and gets the driver from the Platform assigned to the Device.
Maybe just rename it to 'DeviceDriver' in stead of NapalmDriver ?
Other option is to use CustomFields, but it feels like a proper field to retain on the Platform model