Remove MessageSinkManager singleton.

Change-Id: I1cd8a5676beab1f3aba7f62b064dd5b2c2ce0f61
Depends-On: Iaafcea914d16f33ce98c331084cd3e9c8fdbbeda
Reviewed-on: http://gerrit.dmdirc.com/2822
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>