Remove GitHub Pull request templates

[Splines]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This is the current PR template for bug fixes, features etc. During the last year we have never used it for PRs as it oftentimes didn't fit to our changes and the real situation. E.g. most PRs don't introduce "breaking changes" and it's not clear what "breaking" is in the first place as our user base are students who will be presented with a new web UI, so "breaking" could mean that a button was rearranged. Despite not using this template, I feel like we still managed to successfully summarize in PRs what they did and what to watch out for in reviews. That's why we remove the template with this PR such that we don't have to delete the text every time we create a new PR.

Note that we have also added GitHub Actions check in the meantime such that linting is automatically verified in our pipeline. We will do so as well with cypress tests and unit tests in the future, such that we don't have to checkmark it here and can instead rely on real code execution.

• Please check if the PR fulfills these requirements

  • E2E Tests for the changes have been added via Cypress
  • Meaningful rspec tests have been added
  • Docs have been added / updated

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

• Other information: