Remove PropTypes from MaskedViewIOS.ios.js (#21346)

Summary:
related #21342

This is a first PR for this repo.
So, if there are any problem, please tell me 🙇

TODO
* delete props types
* apply read only interface

CheckList
 - [x] `yarn prettier`
 - [x] `yarn flow-check-ios`
Pull Request resolved: https://github.com/facebook/react-native/pull/21346

Differential Revision: D10081962

Pulled By: TheSavior

fbshipit-source-id: 32387c58f180b9aa5f854e323a4bb29aa73f04c8

Libraries/Components/MaskedView/MaskedViewIOS.ios.js

@@ -8,8 +8,6 @@
  * @flow
  */
 
-const DeprecatedViewPropTypes = require('DeprecatedViewPropTypes');
-const PropTypes = require('prop-types');
 const React = require('React');
 const StyleSheet = require('StyleSheet');
 const View = require('View');
@@ -20,16 +18,16 @@ import type {ViewProps} from 'ViewPropTypes';
 
 const RCTMaskedView = requireNativeComponent('RCTMaskedView');
 
-type Props = {
+type Props = $ReadOnly<{|
   ...ViewProps,
 
-  children: any,
+  children: React.Node,
   /**
    * Should be a React element to be rendered and applied as the
    * mask for the child element.
    */
   maskElement: React.Element<any>,
-};
+|}>;
 
 /**
  * Renders the child view with a mask specified in the `maskElement` prop.
@@ -68,11 +66,6 @@ type Props = {
  *
  */
 class MaskedViewIOS extends React.Component<Props> {
-  static propTypes = {
-    ...DeprecatedViewPropTypes,
-    maskElement: PropTypes.element.isRequired,
-  };
-
   _hasWarnedInvalidRenderMask = false;
 
   render() {