Remove support for standalone config files

[damithc]

This feature https://reposense.org/ug/customizingReports.html#get-target-repos-to-provide-more-info is not used that much. In fact, I'm not sure if it is a good idea to let the repo control a report.

Perhaps we can remove it, to ease our code maintenance?

[sopa301]

This will be a breaking change, so the release resolving this issue needs to be a major release.

[ckcherry23]

This change would make #430 irrelevant, as it discusses priority order between standalone config, CLI arguments and csv configs, and standalone configs will not be supported anymore. However, discussions there would still be useful to make a decision regarding priorities for the one-stop config file in #2161 and the csv config files.