remove bridgedb

After checking that everything is fine with the switch to rdsys let's remove bridgedb.

That requires:

• checking with @meskio everything here is ready to be done
• remove bridges@bridges.torproject.org email alias from polyanthum already bouncing
• remove bridges.torproject.org MX record
• remove inbound mail configuration from polyanthum?
• remove bridgedb user and group (from LDAP? note that this requires going through all files, on all affected servers, to find files owned by this user and giving them to another, which user should inherit those?)
• (@meskio) remove /srv/bridges.torproject.org except for bridge descriptors, scripts for syncronization, ...