Remove default color value for mac ActivityIndicator so that it uses (microsoft#23)

the default system color instead.

Author: tom-un

Libraries/Components/ActivityIndicator/ActivityIndicator.js

@@ -116,7 +116,7 @@ const ActivityIndicatorWithRef = React.forwardRef(ActivityIndicator);
 
 ActivityIndicatorWithRef.defaultProps = {
   animating: true,
-  color: Platform.OS === 'ios' || Platform.OS === 'macos' ? GRAY : null, // TODO(macOS ISS#2323203)
+  color: Platform.OS === 'ios' ? GRAY : null,
   hidesWhenStopped: true,
   size: 'small',
 };