refactor: remove unused fetch-lib command

[lengau]

This removes the unused pre-craft-application fetch-lib command.

[maurorodrigues]

hey @lengau, I'm no charm expert, but I've been using fetch-lib. Is there an alternative to fetch-lib? Maybe something automatically done within the charmcraft process?

[lengau]

@maurorodrigues this only applies to an implementation that's no longer used, not to the command itself.

We're moving towards defining the libraries in charmcraft.yaml and using the newer fetch-libs command with that, but we currently have no plans to remove fetch-lib as a charmcraft command available to those who need it.