Delete some superfluous helper functions in HscMain

The driver code is some of the nastiest in GHC, and I am worried about being able to untangle all the tech debt. In HscMain we have a number of helpers which are either not-used or little used. I delete them so we can reduce cognative load, distilling the essential complexity away from the cruft.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.