ProjectExplorer: Remove BuildConfigurationFactory::priority

It's essentially an intrusive hack that's used only to prioritize
Ios+Qmake over plain Qmake. The effect is now achieved by an
arguably equally evil dependency on the construction order of
BuildConfiguration factories.

It can be argued, however, that this is a feature as it allows
user plugins to intentionally override core functionality by
using the standard setup pattern and depending on the to-be-
overridden plugin.

Change-Id: Ic1efa305daf3ca19a880d2a7ccb40e2768d8f57c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmlprofiler/tests/qmlprofilerdetailsrewriter_test.cpp

@@ -85,11 +85,6 @@ public:
     {
         return {};
     }
-
-    int priority(const ProjectExplorer::Kit *, const QString &) const final
-    {
-        return 0;
-    }
 };
 
 QmlProfilerDetailsRewriterTest::QmlProfilerDetailsRewriterTest(QObject *parent) : QObject(parent)