chore: remove legacy architecture documentation (#2752)

This was a mix of architecture and usage guide for legacy librarian
system. Instead this has been replaced by [high level architecture
guide](https://github.com/googleapis/librarian/blob/main/doc/language-onboarding.md#core-concepts)
and [usage
guide](https://github.com/googleapis/librarian/blob/main/doc/library-maintainer-guide.md)

fixes #2744

Author: ldetmer