mon/PaxosService: fix trim completion

Do not call C_Committed after trim or else we will prematurely clear
the bool proposing, propose something again using the same version, and
crash.  We do not in fact need anything to happen here aside from the
refresh_from_paxos() that happens on its own.

Broken by 39b71c582628d26e749c23343aa69db079ee40a1.

Signed-off-by: Sage Weil <sage@inktank.com>