[Mono-winforms-list] Remove horrible MONO_MWF_USE_CARBON_BACKEND env vars

[Miguel de Icaza]

> We could possible provide a env override to force back to X11 on the  
> mac, but since we dont work on 10.5 anyways (x11) I'm not sure how  
> much value there is in this.

Supporting X might come in handy when we do designer integration into
MD.

Miguel