Cleaned up references to executor specific RPCServer types

In oslo.messaging design phase, it was planned to introduce executor
specific RPCServer types though they were never implemented. References
to those types sneaked into docstrings from the original design
document. Since we're not going to implement those types now, docstrings
should be updated accordingly.

Change-Id: Ic4017bb8f89d47115bb7dc59483e81d26c76212b
Merge "Cleaned up references to executor specific RPCServer types"