hide QMAKE_MKSPECS from display of qt version info

BaseQtVersion & co. don't use it for anything anymore. and as the value
used by the project evaluator and qmake during actual project processing
may be "slighly" more extensive than what an out-of-project -query
returns, displaying the value is actually somewhat counterproductive.

Change-Id: I7f22044780d00e71e18d6f3bcf636df756bbcdd3
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>