Remove Msf::TaskManager

MSP-11614

`Msf::TaskManager` was only used for `Msf::DBManager#sink`, which was
removed because it was unused, so `Msf::TaskManager` can also be
removed.

Author: limhoff-r7

lib/msf/core/db_manager.rb

@@ -17,7 +17,6 @@
 require 'msf/core/db_import_error'
 require 'msf/core/host_state'
 require 'msf/core/service_state'
-require 'msf/core/task_manager'
 
 # The db module provides persistent storage and events. This class should be instantiated LAST
 # as the active_suppport library overrides Kernel.require, slowing down all future code loads.