NMAcdManager is a rather simple instance.

It does not need (thread-safe) ref-counting, in fact, having
it ref-counted makes it slighly ugly that we connect a signal,
but never bother to disconnect it (while the ref-counted instance
could outlife the signal subscriber).

We also don't need GObject signals. They have more overhead
and are less type-safe than a regular function pointers. Signals
would make sense, if there could be multiple independent listeners,
but that just doesn't make sense.

Implementing it as a plain struct is less lines of code, and less
runtime over head.

Also drop the possiblitiy to reset the NMAcdManager instance.
It wasn't needed and I think it was buggy because it wouldn't
reset the n-acd instance.

https://github.com/NetworkManager/NetworkManager/pull/213