_Community Builder has loads of plugins that can be computing heavy even when disabled in backend.
This is simply not true. Unpublished plugins only load in backend because in backend they're still accessible, but on frontend they will not load. They don't compute anything when they load and only do something when actually accessed. Most of their main PHP files are just assigning autoloader paths and nothing else.
-> I would like to delete Community Builder Plugins from database of sites WITHOUT deleting the PHP code (as the other type of sites are running that community builder plugin). How can I do that?
Simply put don't do that. Just unpublish them on those sites. If you insist on deleting them then CB plugins can be found in the below table.
_comprofiler_plugin