Remove support for old style build

New style build (aka compiling host-tools as part of the whole project and not separately) is being used for at least a year now, but we still have if(NOT NEW_STYLE_BUILD) and similar in the CMake files.
This blows up our CMake files and creates additional work when adding a host tool (see e.g. /trunk/reactos/CMakeLists.txt:111).

I don't see why anyone should still use old-style build, so can we do a cleanup and get rid of old-style build support altogether?