Vcs: Dissolve VcsManager::registerVersionControl

It's a somewhat unusual pattern, spelling it out will help
to move the vcs plugins setup closer to the now-usual pattern.

Change-Id: Ie7b70e52d75e34cd36b6a1d55ca868e3edab44a7
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/mercurial/mercurialplugin.cpp

@@ -122,7 +122,8 @@ bool MercurialPlugin::initialize(const QStringList & /* arguments */, QString *
     Core::Context context(Constants::MERCURIAL_CONTEXT);
 
     m_client = new MercurialClient;
-    auto vc = initializeVcs<MercurialControl>(context, m_client);
+    auto vc = new MercurialControl(m_client);
+    initializeVcs(vc, context);
 
     new OptionsPage(vc, this);