Remove rx_SetEpoch, rx_SetConnectionEpoch, rx_SetConnectionId

The core RX code now manages the Epoch and CID; external callers
should not be modifying them.

Change-Id: Ia517205aa280b2a0bbd2568274be7fb010fba0bc
Reviewed-on: http://gerrit.openafs.org/10843
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>