Fix stage-qa detection of Qt5 components.

[adridg]

Summary

There used to be a mix of qt5-<foo> and <foo>-qt5 ports all of which
were Qt components. The semantics of ports naming have been rationalized
by kde@, and now qt5-<foo> is the Qt5 component called foo; <foo>-qt5
is a port <foo> that uses Qt5. For instance, wireshark-qt5 is not a
Qt5 component.

Take out the check that suggests that all <foo>-qt5 ports should be
part of USE_QT5, since all of those suggestions are wrong.

Test Plan

Build e.g. deskutils/charmtimetracker on a system with qtkeychain-qt5
installed; you'll need to edit Makefile and remove the CMake argument
that turns off keychain-detection. As part of stage-qa, you will get
a warning saying to add USE_QT5+=qtkeychain .. which then breaks the build.

[rakuco]

[tcberner]

You need an ok from portmgr though.