The generated code is really just a thin wrapper around direct
GDBusProxy calls. GDBusProxy is reasonably convenient to use directly,
drop this wrapper.

We also don't use a generated wrapper for other cases where
NetworkManager acts as a D-Bus client. There is no reason to
do it in this case.

While the nmdbus_*() functions that we were using are small wrappers,
we also created a NMDBusSecretAgent instance, and hence several other
functions and symbols are used as well. It's unnecessary.

This saves 8552 bytes for NetworkManager binary (2817944 vs. 2809392
bytes for contrib/rpm on x86_64).