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.  This removal eliminates [an unnecessary thread](https://github.com/limhoff-r7/metasploit-framework/blob/55a8f6f339111317c62c4bf08ee882403fb5e835/lib/msf/core/task_manager.rb#L124-L130) that [woke up every 1/4 second](https://github.com/limhoff-r7/metasploit-framework/blob/55a8f6f339111317c62c4bf08ee882403fb5e835/lib/msf/core/task_manager.rb#L183) in `Msf::TaskManager`.

# Verification Steps

## specs don't show references to removed code.
- [x] `rake spec`
- [x] VERIFY no failures