refactor: removed macvtap support with firecracker

[richardcase]

What this PR does / why we need it:

This change removes support for macvtap network interfaces when using Firecracker as the microvm provider. It also updates the documentation to state this.

The automation around the Firecracker fork has also been removed.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #856

Special notes for your reviewer:

Checklist:

• [x] squashed commits into logical changes
• [x] includes documentation
• [x] adds unit tests
• [ ] adds or updates e2e tests

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[richardcase]

All checks are passing, so merging.