Autotest: Adapt to changed settings access pattern again

Settings are created on first access now, even though this
doesn't matter much here as this happens unconditionally in
AutotestPluginPrivate ctor via m_frameworkManager.synchronizeSettings()

Change-Id: I340927cf107d7e4b2268e842d23f1a89898e8a92
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/testsettings.h

@@ -27,8 +27,6 @@ class TestSettings : public Utils::AspectContainer, public NonAspectSettings
 public:
     TestSettings();
 
-    static TestSettings *instance();
-
     void toSettings() const;
     void fromSettings();
 
@@ -50,4 +48,6 @@ public:
     RunAfterBuildMode runAfterBuildMode() const;
 };
 
+TestSettings &testSettings();
+
 } // Autotest::Internal