in the c++ module, the .pri files add their directories, so there is
no point in a centralized common.pri doing it again.

the qml plugin doesn't need to add any paths from the c++ module
explicitly, as the QT+=charts-private implies that already.

Change-Id: I263dd702e9874e4d6188ca7251a482d0df4e570b
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>