Remove git submodules

[pcj]

Surprised you'd need this with a bazel build, but maybe there's something on the windows side which is forcing this. Anyway it would be nice not to involve git submodules if possible.

[chrschng]

The submodules are not strictly necessary, but they make it easier to work with the code as most IDEs just find the dependencies at the expected directories. That's why we decided to keep the most prevalent dependencies as submodules (absl, proto, gprc, googletest).

What's the problem with submodules?

[ljusten]

In Visual Studio we add custom include directories in project files, so http archives can be made discoverable, and Intellisense works. It has a small caveat that VS Intellisense doesn't always work well with Bazel. Bazel is super heavy on symlinks that are created based on BUILD files and Intellisense doesn't always update when symlinks change.

One issue with submodules is that git submodule init takes 3-4 minutes in the Github workflows. I'm not sure whether that would get any better if they were implemented as http archives. Maybe the load would be moved to bazel build.

[ljusten]

Another issue with submodules is that it seems to make git operations like git status very slow, probably because it looks into the submodules as well.