Make pop_frame static

This function is only used within the same cpp file.
Making it static simplifies the global interface and is potentially more efficient.

Author: romainbrenguier

src/goto-symex/goto_symex.h

@@ -272,8 +272,6 @@ class goto_symext
   virtual void loop_bound_exceeded(statet &, const exprt &guard);
 
   // function calls
-
-  void pop_frame(statet &);
   void return_assignment(statet &);
 
   virtual void no_body(const irep_idt &)

src/goto-symex/symex_function_call.cpp

@@ -327,7 +327,7 @@ void goto_symext::symex_function_call_code(
 }
 
 /// pop one call frame
-void goto_symext::pop_frame(statet &state)
+static void pop_frame(goto_symext::statet &state)
 {
   PRECONDITION(!state.call_stack().empty());