Don't hardcode utf8 in the sql install file

Low priority since what's there and what those tables do are unlikely to cause any issues, but at some point should think about utf8mb4 so that there isn't a mismatch with core tables.

Also while the utf8conversion core api call allows you to specify additional tables to convert, it doesn't have a hook for extensions to autoadd their own tables so people need to know what the extension tables look like (maybe that could be a core issue).