Remove un-used data from discussions endpoint
When we request discussions.json on merge request it will return the diff file, which some of that data is required. However, also included in that data is the full highlighted_diff_lines & parallel_diff_lines, which on the discussions tab isn't required. Testing locally, this took a response from 6.6mb to 1.1mb.
We could also probably remove image_diff_html seeing as we now render that fully on the frontend.