Remove unused isAddressInUse helper in anvil module

[mazurroman]

Summary

• `isAddressInUse` and its only call site in `spawnAnvil` were commented out; the function and its `createServer` import were unreferenced. Deletes the helper, the unused import, and the commented-out call block.

Test plan

• `grep -rn "isAddressInUse" src` returns no other usages
• `createServer` from `node:net` is not used elsewhere in the file
• No behavior change — removed code was already inert