Revert "improve abstraction for AssocList, fix bug in optional else (#315)" (#320)

This reverts commit bf6bb800c4.

executable_semantics/main.cpp

@@ -5,7 +5,6 @@
 #include <cstdio>
 #include <cstring>
 #include <iostream>
-#include <string>
 
 #include "executable_semantics/syntax_helpers.h"
 
@@ -24,9 +23,5 @@ int main(int argc, char* argv[]) {
       return 1;
     }
   }
-
-  if (argc > 2 && argv[2] == std::string("-trace")) {
-    Carbon::tracing_output = true;
-  }
   return yyparse();
 }