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/ios/iosbuildconfiguration.cpp

@@ -145,7 +145,6 @@ IosBuildConfigurationFactory::IosBuildConfigurationFactory()
     registerBuildConfiguration<IosBuildConfiguration>(QmakeProjectManager::Constants::QMAKE_BC_ID);
     addSupportedTargetDeviceType(Constants::IOS_DEVICE_TYPE);
     addSupportedTargetDeviceType(Constants::IOS_SIMULATOR_TYPE);
-    setBasePriority(1);
 }
 
 } // namespace Internal