All Plugins: Use global future synchronizer

Instead of using plugin's own synchronizers. The global
synchronizer does the synchronization just before all the
plugins' destructors run (in sync), so this should be
the right equivalent.

Change-Id: I8d09c9ea4a11b7a703684ad5319191ce310d992e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/vcsbase/vcsplugin.h

@@ -11,8 +11,6 @@ QT_BEGIN_NAMESPACE
 class QStandardItemModel;
 QT_END_NAMESPACE
 
-namespace Utils { class FutureSynchronizer; }
-
 namespace VcsBase {
 
 class VcsBaseSubmitEditor;
@@ -36,8 +34,6 @@ public:
 
     CommonVcsSettings &settings() const;
 
-    static Utils::FutureSynchronizer *futureSynchronizer();
-
     // Model of user nick names used for the submit
     // editor. Stored centrally here to achieve delayed
     // initialization and updating on settings change.