update: remove root command, cleanup tests

General MR

Summary

In prep for future work (auto updating bbctl and beyond), clean up the old, unused root command for bbctl update.

Relevant logs/screenshots

danieldides@BPH-MAC14 ~/P/b/p/bbctl (dd/cleanup-bbctl-update)> bbctl update
msg: No update functionality has been implemented yet

Linked Issue

closes #296

Upgrade Notices

N/A