Remove unused methods of smt2_conv

No functional change.
This functions were not used anyway, this simplifies the interface.

Author: romainbrenguier

src/solvers/smt2/smt2_conv.cpp

@@ -4092,11 +4092,6 @@ void smt2_convt::unflatten(
   }
 }
 
-void smt2_convt::convert_overflow(const exprt &)
-{
-  UNREACHABLE;
-}
-
 void smt2_convt::set_to(const exprt &expr, bool value)
 {
   PRECONDITION(expr.type().id() == ID_bool);

src/solvers/smt2/smt2_conv.h

@@ -148,7 +148,6 @@ class smt2_convt:public prop_convt
   // tweaks for arrays
   bool use_array_theory(const exprt &);
   void flatten_array(const exprt &);
-  void unflatten_array(const exprt &);
 
   // specific expressions go here
   void convert_byte_update(const byte_update_exprt &expr);
@@ -172,15 +171,12 @@ class smt2_convt:public prop_convt
   void convert_mod(const mod_exprt &expr);
   void convert_index(const index_exprt &expr);
   void convert_member(const member_exprt &expr);
-  void convert_overflow(const exprt &expr);
+
   void convert_with(const with_exprt &expr);
   void convert_update(const exprt &expr);
 
   std::string convert_identifier(const irep_idt &identifier);
 
-  // introduces a let-expression for operands
-  exprt convert_operands(const exprt &);
-
   // auxiliary methods
   void find_symbols(const exprt &expr);
   void find_symbols(const typet &type);