Drop unused device-level mac_prefix custom field

Summary

• Removes DeviceSpec.MACPrefixOverride and the NetBox mac_prefix custom-field read — it was plumbed in but never consumed by diff.Compute, which only ever used the global RAX_MAC_PREFIX_DEFAULT.
• Explicit per-interface MAC via primary_mac_address is unaffected — that stays the real override path. Hop179 has one MAC convention across both nodes, so a per-device OUI override isn't a real requirement here.
• Also updates the build plan doc to match: mac_prefix is no longer a NetBox custom field to create.

Test plan

• go build ./..., go vet ./..., gofmt -l . clean
• go test ./... green (netbox package fixtures updated to drop mac_prefix from custom_fields JSON)

🤖 Generated with Claude Code

https://claude.ai/code/session_01U9buZPRQy7YXLodkSPFFik