Optimize remove_level_2

Instead of building the identifier from scratch, just remove the l2 suffix.

Author: romainbrenguier

src/util/ssa_expr.cpp

@@ -99,5 +99,8 @@ std::pair<irep_idt, irep_idt> ssa_exprt::build_identifier(
 void ssa_exprt::remove_level_2()
 {
   remove(ID_L2);
-  update_identifier();
+  const std::string &identifier = id2string(get_identifier());
+  auto index = identifier.find_last_of('#');
+  if(index != std::string::npos)
+    set_identifier(identifier.substr(0, index));
 }