[es6] omit callable check if possible, since %_Call already does it.

src/js/string.js

Index: src/js/string.js
diff --git a/src/js/string.js b/src/js/string.js
index c838064112f4543dda4e9dc2fb97c0b55d5f6bd3..139bf7a654524aec8060dd3fc6689de60f6655b1 100644
--- a/src/js/string.js
+++ b/src/js/string.js
@@ -162,9 +162,6 @@ function StringMatchJS(pattern) {
   if (!IS_NULL_OR_UNDEFINED(pattern)) {
     var matcher = pattern[matchSymbol];
     if (!IS_UNDEFINED(matcher)) {
-      if (!IS_CALLABLE(matcher)) {
-        throw MakeTypeError(kCalledNonCallable, matcher);
-      }
       return %_Call(matcher, pattern, this);
     }
   }
@@ -572,9 +569,6 @@ function StringSplitJS(separator, limit) {
   if (!IS_NULL_OR_UNDEFINED(separator)) {
     var splitter = separator[splitSymbol];
     if (!IS_UNDEFINED(splitter)) {
-      if (!IS_CALLABLE(splitter)) {
-        throw MakeTypeError(kCalledNonCallable, splitter);
-      }
       return %_Call(splitter, separator, this, limit);
     }
   }