How do I remove the whole "Recent Topics" widget from the main page?

Look at the templates, do you have modern layout? You can remove the relevant parts from there.

You can also set $viewlastdiscussions var under setup_options.php to zero to disable this block from parsing.
If this block will not be present, most likely you should also switch to the old-school layout of forums, turning $startPageModern=FALSE;