realtime: atomic control: don't push/pull string controls

It's not real-time safe (yet?), so disable for now.
realtime: remove unused API (reduce maintanance cost)

AtomicControl is only used to get values from network, not set them.
Arguably, it's not even useful for bidirectional transport.
Therefore, remove part of it's API for setting control values.
realtime: support getting/setting realvec controls