Remove or Improve "Build Environment" section in Operating Systems and Supported Platforms

In contrast to the other sections in "Operation Systems and Supported Platforms" page, the "Build Environment" is about what you need to build QtCreator yourself. However, the two points listed there are too little, and not even correct (IMO we require 4.7.1 now). E.g. one would have to explain the private header problems, where to get MinGW 4.4 from etc.

I personally would just leave it out: We have README in the sources that are for the curious, if you see the help you've already creator running.