Don't skip looking for duplicate glob->mimetype associations.

Skipping doesn't simply create a performance problem as I thought,
it actually creates a fake "ambiguity" ("more than one mimetype found for
this glob"), which leads to determination-from-contents.

Testcase:
kmimetypefinder kdeplasma-addons/applets/nowplaying/package/contents/ui/PositionSlider.qml
(any QML file that starts with a C-like comment)
with two lines "50:text/x-qml:*.qml" in /usr/share/mime/globs2.

I'll add a unit test in Qt 5 and fix the bug there as well.
Then I'll look into fixing the same bug in xdg-mime itself...