build system cleanups

- load(qt_module) => load(qt_build_config)
- remove:
  - CONFIG+=module (obsolete)
  - setup of PKGCONFIG* variables (automated now)
  - code relating to module version headers (automated now)
  - %mastercontent assignment (automated now)
  - QT_BUILD_*_LIB defines (automated now)
  - pointless QPRO_PWD assignments

Change-Id: Ie8a638b3c3a1a6a3d37fc410435bbc2b4a068ecf
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>