Get rid of IPROTO_SERVER_IS_RO

Not needed anymore as we now use the new IPROTO_VOTE command instead of
IPROTO_VOTE_DEPRECATED. Let's remove it altogether and reuse its code
for IPROTO_BALLOT (they are never decoded together so no conflict should
happen). Worst that can happen is we choose a read-only master when
bootstrapping an older version of tarantool.