Utils: Re-do Replace SynchronousCommand::flags by a named bool

The potential extensibility was never used, and a similar use case
in QtcProcess (setLowPriority()) used a named setter.

Change-Id: I87d84ea2206995d5069265f8df78f40cf2ba8a36
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcscommand.h

@@ -51,7 +51,6 @@ public:
                                                  const Utils::ExitCodeInterpreter &interpreter = Utils::defaultExitCodeInterpreter) override;
 
 private:
-    unsigned processFlags() const override;
     void emitRepositoryChanged(const QString &workingDirectory);
 
     void coreAboutToClose() override;