BACKENDS: Remove old mingw taskbar compaibility header

Mingw now has this functionality built in. I would point you to the commit where it happened, but it looks to me like they added little bits over time. This change is needed for me to build with the mingw32 that is in the Fedora 19 repositories. 

The main issue is that any old build bots or old setups are probably dependant on the current functionality. Maybe it's time to give them an update anyway.