Remove rules_gwt from Bazel CI

[philwo]

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.

[philwo]

@aiuto @sventiffe Any thoughts or concerns?

[comius]

I second that.

[Wyverald]

The project is now disabled on Bazel CI; I'll leave this issue open to track actually removing it.