Add opt-in flag for buf generate to delete out dir

[weeco]

When running buf generate I usually run rm -rf before, so that deleted proto code will not be forever stale in your output directory. When using buf's workspaces and having multiple modules it would be nice if you can rely on buf deleting these output directories, as the buf cli is already aware of the output directories.

Since this may not be the desired behaviour for everyone I'm proposing to make an opt-in flag/config for this.

[bufdev]

Yea we should definitely do this. This annoys me too.