Remove dead code

The front-ends handle type conversion when doing arithmetic over enum types. The
case of an enum tag is not expected here.

Author: tautschnig

src/goto-programs/goto_convert_side_effect.cpp

@@ -105,19 +105,10 @@ void goto_convertt::remove_assignment(
       tmp.op0().make_typecast(expr.op1().type());
       rhs=typecast_exprt(is_not_zero(tmp, ns), expr.op0().type());
     }
-    else if(op0_type.id() == ID_c_enum_tag)
-    {
-      // We convert c_enums to their underlying type, do the
-      // operation, and then convert back
-      const auto &enum_type = ns.follow_tag(to_c_enum_tag_type(op0_type));
-      auto underlying_type = enum_type.subtype();
-      auto op0 = typecast_exprt(expr.op0(), underlying_type);
-      auto op1 = typecast_exprt(expr.op1(), underlying_type);
-      binary_exprt tmp(op0, new_id, op1, underlying_type);
-      rhs = typecast_exprt(tmp, expr.op0().type());
-    }
     else
     {
+      PRECONDITION(
+        op0_type.id() != ID_c_enum_tag && op0_type.id() != ID_c_enum);
       rhs.id(new_id);
       rhs.copy_to_operands(expr.op0(), expr.op1());
       rhs.type()=expr.op0().type();