Remove Helmet LTI Dependency

[lkiesow]

NPM started complaining about conflicting dependencies in the LTI
module, potentially causing build failures.

The reason for the conflicts was the dependency Helmet, which hasn't
been updated by us for quite some time and also wasn't really used.

This patch removes that dependency, fixing the build process.

…and down the rabbit hole I went. This now also fixes the other issues breaking th build:

• The six years old mock proxy did no longer load its configuration. Fixed in f2adaeb
• The autoprefixer caused a deprecation warning, which killed the CI build. Fixed in c412a3f

Your pull request should…

• have a concise title
• close an accompanying issue if one exists
• be against the correct branch
• include migration scripts and documentation, if appropriate
• pass automated tests
• have a clean commit history
• have proper commit messages (title and body) for all commits

[lkiesow]

That's weird and new…

 > opencast-lti@0.1.0 mock-proxy
> npx http-mock-server --config mock-server-config.json --proxy http://localhost:3000
No config file path set.
Mock server listening on port 8888

• The server should listen on 7878 (that's why the tests fail)
• the config is passed to the tool?