FindQHelpGenerator: avoid picking up Qt4 version

Summary:
Passing NO_DEFAULT_PATH ignores $PATH and ensures that we use the
previously-detected Qt5 binary path.

Test Plan:
qhelpgenerator is now picked up from the same location as Qt5::qmake. Before,
anything in $PATH was preferred even if it was the Qt 4 version.

Reviewers: Frameworks, kossebau, kfunk

Reviewed By: kossebau, kfunk

Subscribers: alexeymin, asturmlechner, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D6249