Remove UI

[milenkovicm]

As part of effort outlined in #1066 and #1067 this PR removes UI package from core ballista.

Relates to: #1066 & #1067

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[andygrove]

This looks good. Thanks.

Could you also update the documentation (user guide, contributor guide, etc)?

Here is a partial list of references:

$ find . -name "*.md" -exec grep -Hi "web ui" {} \;
./docs/source/contributors-guide/code-organization.md:- [Web UI](https://github.com/apache/datafusion-ballista/tree/main/ballista/scheduler/ui)
./docs/source/user-guide/introduction.md:![Ballista Scheduler Web UI](./images/ballista-web-ui.png)
./docs/source/user-guide/scheduler.md:![Ballista Scheduler Web UI](./images/ballista-web-ui.png)
./docs/source/user-guide/deployment/docker.md:Starting nginx to serve Ballista Scheduler web UI on port 80
./docs/source/user-guide/deployment/docker-compose.md:The scheduler web UI is available on port 80 in the scheduler.
./CONTRIBUTING.md:- Javascript (Scheduler Web UI)

[milenkovicm]

should be removed now, plus few extra lines i found

[andygrove]

LGTM. Could you fix the merge conflict?

[milenkovicm]

done

[andygrove]

Looks like there is still a reference in the docker scripts:

+ cd ballista/scheduler/ui
/home/builder/builder-entrypoint.sh: line 27: cd: ballista/scheduler/ui: No such file or directory