fix: remove plan drive type and nic type enums (#156)

The [previous fix for plan drive
type](https://github.com/equinix-labs/metal-go/155) did not work; it
turns out that the API plan data includes plans with drive type `NVMe`
_and_ plans with drive type `NVME`. As mentioned on the previous PR,
adding both options to the enum generates invalid code, and we don't
have a practical way to enable case-insensitive comparisons for string
enums.

Reverting the plan drive type to a plain string uncovered a different
issue with the nic type enum: some plans (`storage` type, possibly
others) include the following for drives and nics:

```json
                "drives": [
                    {
                        "count": 0,
                        "size": "",
                        "type": ""
                    }
                ],
                "nics": [
                    {
                        "count": 0,
                        "type": ""
                    }
                ]
```

Rather than test `nullable: true` here, I opted to change the nic type
to a plain string as well.

API.md

@@ -570,11 +570,9 @@ Class | Method | HTTP request | Description
  - [PlanSpecsCpusInner](docs/PlanSpecsCpusInner.md)
  - [PlanSpecsDrivesInner](docs/PlanSpecsDrivesInner.md)
  - [PlanSpecsDrivesInnerCategory](docs/PlanSpecsDrivesInnerCategory.md)
- - [PlanSpecsDrivesInnerType](docs/PlanSpecsDrivesInnerType.md)
  - [PlanSpecsFeatures](docs/PlanSpecsFeatures.md)
  - [PlanSpecsMemory](docs/PlanSpecsMemory.md)
  - [PlanSpecsNicsInner](docs/PlanSpecsNicsInner.md)
- - [PlanSpecsNicsInnerType](docs/PlanSpecsNicsInnerType.md)
  - [PlanType](docs/PlanType.md)
  - [Port](docs/Port.md)
  - [PortAssignInput](docs/PortAssignInput.md)