build!: remove Jenkins CI scripts (#31812)

These files were used to run tests on
build.testeng.edx.org, a Jenkins instance.

All tests have since been moved over to
GitHub Actions. The build.testeng.edx.org
instance is decomissioned.

Removed files include:
- scripts/jenkins-*.sh
- scripts/Jenkinsfiles/*
- scripts/xdist/*

Note on xdist scripts: pytest-xdist in general is still
useful for local test runs in order to take advantage
of all available CPU cores, but the scripts here were
specifically for running tests on multiple remote machines,
particularly via Jenkins.