fix: remove @available for RCTKeyWindow check (#46510)

Summary:
This PR removes `available` for `RCTKeyWindow` after minimum version bump to iOS 15 this check is not needed anymore.

## Changelog:

[IOS] [REMOVED] - remove available for RCTKeyWindow check

Pull Request resolved: #46510

Test Plan: CI Green

Reviewed By: rshest

Differential Revision: D62752434

Pulled By: javache

fbshipit-source-id: 7b7745d4f27128cdb66053f7fa1cc234435eac31

packages/react-native/React/Base/RCTUtils.m

@@ -586,17 +586,7 @@ BOOL RCTRunningInAppExtension(void)
   UIScene *sceneToUse = foregroundActiveScene ? foregroundActiveScene : foregroundInactiveScene;
   UIWindowScene *windowScene = (UIWindowScene *)sceneToUse;
 
-  if (@available(iOS 15.0, *)) {
-    return windowScene.keyWindow;
-  }
-
-  for (UIWindow *window in windowScene.windows) {
-    if (window.isKeyWindow) {
-      return window;
-    }
-  }
-
-  return nil;
+  return windowScene.keyWindow;
 }
 
 UIStatusBarManager *__nullable RCTUIStatusBarManager(void)