KPluginFactory: Refactor and simplify internals

- Only use one function type and discard the metadata if needed.
 This way we only need one internal container, which saves us memory.
- Remove keyword leftovers, all of that has been deprecated
- Use simple std::vector to store registrations
- Use simple d-ptr instead of Q_D macro. We do not have the case where
 we extend the private class.
 Also, there isn't any logic which heavily depends on const-correctness.
 Thus we can simplify the code.

Task: https://phabricator.kde.org/T14952