Remove rules_gwt from Bazel CI

https://github.com/bazelbuild/rules_gwt has been broken for a long time, because it's apparently unmaintained and an incompatibility with Java >= 9 is not being addressed.

We attempted to fix this ourselves (https://github.com/bazelbuild/rules_gwt/pull/38) but unfortunately it seems to be not that easy.

Let's remove the project from Bazel CI and I also propose to archive the repository and mark it as unmaintained.