Remove replaceOkHttpClient method

replaceOkHttpClient is a noop since
0a71f48.

Leaving this code in place is dangerous as it can lead to subtle bugs
where people upgrading react-native assume they're continuing to replace
the okhttp client when in fact this is not the case.

ReactAndroid/src/main/java/com/facebook/react/modules/network/OkHttpClientProvider.java

@@ -46,12 +46,6 @@ public static OkHttpClient getOkHttpClient() {
     }
     return sClient;
   }
-
-  // okhttp3 OkHttpClient is immutable
-  // This allows app to init an OkHttpClient with custom settings.
-  public static void replaceOkHttpClient(OkHttpClient client) {
-    sClient = client;
-  }
 
   public static OkHttpClient createClient() {
     if (sFactory != null) {