Fix signature of blocking Proxy::call_with_flags

The flags arg to call_with_flags is no longer generic, but it still had the generic type parameter for it in its signature, so the code generated by the dbus_proxy macro ended up generating calls that weren't sufficiently qualified. The existing e2e test didn't catch this because it didn't generate the blocking proxy

Removed the unused type param and changed the e2e test to generate the blocking proxy, even if it's not used, to exercise the codegen