Removed states-xtext example

[spoenemann]

The Xtext-based example doesn't work properly (#106 (review)), but we don't have the resources to keep maintaining it. The same functionality is now fully provided by the Langium-based example. Removing the older example also brings advantages for the project:

• Less overall complexity (only one programming language and build system vs. two)
• Faster build time (previous: 67s Java build + 42s Yarn build; now: 24s Yarn build – tested in Gitpod)
• Fewer dependencies and lower maintenance effort

[dhuebner]

@spoenemann
It would be nice to have a tag 'xtext-example' (or a branch) that points to a commit that contains removed code. It could be helpful for people who want to interact with Xtext. WDYT?

[dhuebner]

Less code less maintenance.

[spoenemann]

I pushed a tag xtext-example.