Remove propTypes checks for legacy context (#28324)

Part of #28207, this is easy to
land in isolation.

The approach I'm taking is slightly different — instead of leaving
validation on for legacy context, I disable the validation (it's
DEV-only) and leave just the parts that drive the runtime logic. I.e.
`contexTypes` and `childContextTypes` *values* are now ignored, the keys
are used just like before.

DiffTrain build for [64e755d](64e755d)

Author: gaearon

compiled/facebook-www/REVISION

@@ -1 +1 @@
-5d445b50a670d6cdc52943ca4e5ab3422792c57c
+64e755d7374d753c75dc16ba1a4a09ce7b8689dd

compiled/facebook-www/React-prod.classic.js

@@ -618,4 +618,4 @@ exports.useSyncExternalStore = function (
 exports.useTransition = function () {
   return ReactCurrentDispatcher.current.useTransition();
 };
-exports.version = "18.3.0-www-classic-a886996f";
+exports.version = "18.3.0-www-classic-e7aae6cc";