VCS[vcs,p4,svn,hg]: Use VCManager for delete operations.

...and its dialog prompt consistently. Rename method and
introduce overload taking IVersionControl. Add convenience slot to
VCSBasePlugin.

src/plugins/cvs/cvsplugin.h

@@ -99,7 +99,6 @@ public:
 
 private slots:
     void addCurrentFile();
-    void deleteCurrentFile();
     void revertCurrentFile();
     void diffProject();
     void diffCurrentFile();