fix: remove category enum from plan drives spec

[ctreatma]

Closes #97

[ctreatma]

There's a weird CI failure in here, and I'm at a loss as to why it's happening.

On an ansible-collection-equinix PR to upgrade metal-python to 0.10.0, the only test failure is the enum validation issue that this PR addresses: https://github.com/equinix-labs/ansible-collection-equinix/actions/runs/9407693322/job/25914037577?pr=191

On this PR, which is based on 0.10.0 (so the only code change here compared to that version is the removal of one enum validator), a different test is failing consistently: https://github.com/equinix-labs/metal-python/actions/runs/9407997426/job/25914997489?pr=98#step:10:3843

I'm leaning towards writing off that test failure as some sort of flake, since it makes no sense that removing an enum from Metal plans would impact the ability to look up a VRF attached to a virtual circuit.

[github-actions]

This PR is included in version 0.10.1 🎉