Remove unused methods in std::string StringTraits

IsNull and SetToNull aren't needed as the STL string type doesn't use
nullable_is_same_type.

Change-Id: I0770b42e275e3026749c5409015c4b79941ea56f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386744Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#803654}

mojo/public/cpp/bindings/string_traits_stl.h

@@ -13,16 +13,6 @@ namespace mojo {
 
 template <>
 struct StringTraits<std::string> {
-  static bool IsNull(const std::string& input) {
-    // std::string is always converted to non-null mojom string.
-    return false;
-  }
-
-  static void SetToNull(std::string* output) {
-    // std::string doesn't support null state. Set it to empty instead.
-    output->clear();
-  }
-
   static const std::string& GetUTF8(const std::string& input) { return input; }
 
   static bool Read(StringDataView input, std::string* output) {