context: we are removing unused branch flag as part of cleanup for go/librarian:cli-reimagined https://github.com/googleapis/librarian/pull/811/files#r2195689048

Support for branches other than main is needed for Java LTS branches.

This might be either:

• making sure Librarian always uses input repo url (googleapis and language repo) and not checkout main branch
• allow a branch parameter and respect that