Drop plugin-based new user welcome.

[dannon]

This didn't work out like we wanted it to, went stale quickly, and was too fragile (currently broken). We still need a new user welcome but this could be explored again in the context of help mode, integrated tutorials a-la GTN, etc.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[mvdbeek]

Thank you!