Clean up prefix flag handling. First of all the dynamic networks no longer

Clean up prefix flag handling. First of all the dynamic networks no longer
need this and are now treated equally to the network statement in the config.
This makes bgpctl network delete <net> also remove a network which was defined
in the config.
While there remove the other use of flag which was done to support Adj-RIB-Out
but the direction we're taking is no longer needing that. Makes code simpler
again.
OK benno@