[ReactNative] ErrorUtils.reportError shouldn't redbox

Summary:
@public

ErrorUtils.reportError is intended for reporting handled errors to the
server, like timeouts, which means that we shouldn't shove them in the
developer's face.

Test Plan: add `require('ErrorUtils').reportError(new Error('error'))` and see a useful error message with stack but no redbox.  Debugger confirms `reportSoftException` is called but it doesn't do anything yet.  `reportFatalError` and throwing exceptions still show redboxes.

React/Modules/RCTExceptionsManager.m

@@ -44,8 +44,7 @@ RCT_EXPORT_METHOD(reportSoftException:(NSString *)message
     [_delegate handleSoftJSExceptionWithMessage:message stack:stack];
     return;
   }
-
-  [[RCTRedBox sharedInstance] showErrorMessage:message withStack:stack];
+  // JS already logs the error via console.
 }
 
 RCT_EXPORT_METHOD(reportFatalException:(NSString *)message