Make goto_cc_modet::main non-virtual

Making this member function non-virtual makes it easier to follow the
control flow because it is clear it not overridden in any sub class.
This is should be easy enough to add back should a need to override this
arise.

Author: thomasspriggs

src/goto-cc/goto_cc_mode.h

@@ -21,7 +21,7 @@ Date: June 2006
 class goto_cc_modet:public messaget
 {
 public:
-  virtual int main(int argc, const char **argv);
+  int main(int argc, const char **argv);
   virtual int doit()=0;
   virtual void help_mode()=0;
   void help();