Remove ExprBuiltin

It's slower than ExprVar since it doesn't compute a static
displacement. Since we're not using the throw primop in the
implementation of <...> anymore, it's also not really needed.

src/libexpr/nixexpr.cc

@@ -123,11 +123,6 @@ void ExprOpNot::show(std::ostream & str)
     str << "! " << *e;
 }
 
-void ExprBuiltin::show(std::ostream & str)
-{
-    str << "builtins." << name;
-}
-
 void ExprConcatStrings::show(std::ostream & str)
 {
     bool first = true;
@@ -342,10 +337,6 @@ void ExprOpNot::bindVars(const StaticEnv & env)
     e->bindVars(env);
 }
 
-void ExprBuiltin::bindVars(const StaticEnv & env)
-{
-}
-
 void ExprConcatStrings::bindVars(const StaticEnv & env)
 {
     foreach (vector<Expr *>::iterator, i, *es)