Omit sources from generated Javadoc

[cpovirk]

When we build Javadoc, we set linksource. That causes the Javadoc tool to generate links from text like public interface Multimap in https://guava.dev/Multimap. Those links go to pages like https://guava.dev/releases/snapshot-jre/api/docs/src-html/com/google/common/collect/Multimap.html#line-164.

I somewhat suspect that no one cares about these pages: If they want source, they'll look in GitHub or in the source jar for a release or snapshot.

Additionally, these pages force the generated Javadoc to change even for implementation changes (example output changes from input changes).

And any additional page presumably slows down downloading history, switching branches, and deploying the guava.dev. site.

I'm not sure I'll even bother to remove linksource, but I'm thinking about it, so let me know if that would cause a problem for you. If I do try it, and if it goes well, I might even consider removing some of the existing sources....

[cpovirk]

(If we cared enough, I'm sure we could rewrite the links to point to GitHub.)