Remove virtual from cover_instrumenter_baset::operator()

That this change compiles shows that this member function is not
specialised in any derived class. This means that we can make further
changes to its signature and implementation without considering derived
classes.

Author: thomasspriggs

src/goto-instrument/cover_instrument.h

@@ -40,7 +40,7 @@ class cover_instrumenter_baset
   /// \param function_id: name of \p goto_program
   /// \param goto_program: a goto program
   /// \param basic_blocks: detected basic blocks
-  virtual void operator()(
+  void operator()(
     const irep_idt &function_id,
     goto_programt &goto_program,
     const cover_blocks_baset &basic_blocks) const