dispose of MSVCPROJ_LIBS

dispose of MSVCPROJ_LIBS

this way QMAKE_LIBS{,_PRIVATE} can be treated the same as in all other
generators, which allows us to:
- make the windows generators' findLibraries() be more like the unix
  version
- dispose of QMAKE_INTERNAL_PRL_LIBS handling while reading prl files
  (because the output always goes to QMAKE_LIBS)
- as a side effect, QMAKE_LIBS_PRIVATE are not subjected to prl file
  resolution any more, which is again consistent with unix - the
  correctness of that needs to be assessed separately.

Change-Id: Ie9bc04d117eff6a7cde846677f98acf3c64aa6ee
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>