Remove unused Msf::DBManager::Sink and Msf::TaskManager

MSP-11614

`Msf::DBManager::Sink` contains a `#sink` that is a `Msf::TaskManager` for serializing database events, but it is currently unused because `ActiveRecord` directly handles multithread access to PostgreSQL with no need for manual serialization.  `Msf::DBManager::Sink` should be removed because starting the `Msf::TaskManager` spawns a thread which is consume resource unnecessarily.  Once the `Msf::DBManager::Sink#sink` was removed, then `Msf::TaskManager` was removed.

# Verification Steps

## specs don't show references to removed code.
- [ ] `rake spec`
- [ ] VERIFY no failures