Remove 'Types of changes' section in MR template

I don't find the 'types of changes' section (bug fix, feature, refactoring, etc.) useful. In many cases, it's obvious from the MR title what category it belongs to, and when it's not, the categories are debatable and thinking about it just makes opening an MR more work.

I'm not sure we're doing anything with this information, but if we want to have a way to systematically classify MRs, Gitlab labels seem better. You can use them for filtering, and it's much more convenient to change labels if the scope of an MR changes or we decide another category fits it better.