VCS: Replace AbstractCheckoutJob with Command

Command now provides the same functionality. Deduplicate code.

Change-Id: I789f021050471281870b6ef6a81a94e66fbdf0c7
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/cvs/cvsplugin.cpp

@@ -225,6 +225,7 @@ bool CvsPlugin::initialize(const QStringList &arguments, QString *errorMessage)
 
     using namespace Core::Constants;
     using namespace ExtensionSystem;
+    using Core::Command;
 
     initializeVcs(new CvsControl(this));