mon/MonmapMonitor: remove some dead code

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: go active *after* refreshing

The update_from_paxos() methods occasionally like to trigger new activity.
As long as they check is_readable() and is_writeable(), they will defer
until we go active and that activity will happen in the normal callbacks.

This fixes the problem where we active but is_writeable() is still false,
triggered by PGMonitor::check_osd_map().

Signed-off-by: Sage Weil <sage@inktank.com>
mon: simplify Monitor::init_paxos()

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