Remove dependency on qhelpgenerator.

At some point "make docs" command started to need qhelpgenerator which
doesn't exist in qtbase, so it can not be used. That was causing
the qdoc bot to fail with a cryptic message:

    make[3]: *** Waiting for unfinished jobs....

Change-Id: I2bf2dd421fc55720a68a4659ddffbd3b789facf1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>