Back out "[react-native][PR] [iOS] Add convert compatible of NSString for bridge message data"

Summary:
Original commit changeset: 4849a8e94141

# Problem:
This diff caused https://github.com/facebook/react-native/issues/25339.
It was converting non string values to strings whenever the conversion was available.

The original problem that this diff was trying to solve is described here -> https://twitter.com/estevao_lucas/status/1117572702083190785?s=215

# Solution introduced by this diff
This diff introduced a conversion. So for any value that was meant to be string but wasn't we would attempt to convert it. Which is not always wanted.

# Solution for problem in the twitter
I think we should improve the error message if incorrect type is being passed as a prop. However this isn't a problem if views are Flow typed.

Reviewed By: cpojer

Differential Revision: D15964298

fbshipit-source-id: 3d027a378561b87e7a9e89fc5e39dc6f359d8d22

React/Base/RCTConvert.m

@@ -53,21 +53,12 @@ RCT_NUMBER_CONVERTER(NSUInteger, unsignedIntegerValue)
 
 RCT_JSON_CONVERTER(NSArray)
 RCT_JSON_CONVERTER(NSDictionary)
+RCT_JSON_CONVERTER(NSString)
 RCT_JSON_CONVERTER(NSNumber)
 
 RCT_CUSTOM_CONVERTER(NSSet *, NSSet, [NSSet setWithArray:json])
 RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncoding])
 
-+ (NSString *)NSString:(id)json
-{
-  if ([json isKindOfClass:NSString.class]) {
-    return json;
-  } else if (json && json != (id)kCFNull) {
-    return [NSString stringWithFormat:@"%@",json];
-  }
-  return nil;
-}
-
 + (NSIndexSet *)NSIndexSet:(id)json
 {
   json = [self NSNumberArray:json];