Remove duplicate implementations of string conversion methods.

BUG=webrtc:4773

Review-Url: https://codereview.webrtc.org/2583853002
Cr-Commit-Position: refs/heads/master@{#15957}

webrtc/sdk/objc/Framework/Classes/helpers.h

@@ -20,6 +20,7 @@ namespace ios {
 
 bool CheckAndLogError(BOOL success, NSError* error);
 
+NSString *NSStringFromStdString(const std::string& stdString);
 std::string StdStringFromNSString(NSString* nsString);
 
 // Return thread ID as a string.