📙 Android client push removed

[TomWFox]

Link to section:
https://docs.parseplatform.org/android/guide/#push-notifications

What is the issue?
The push notification is significantly outdated - in particular in relation to the removal of the option for client push on Parse Server.

Can you propose a solution?
The sections that explain how to send pushes from the client SDK should be removed and it should be explained that this can only be done with the masterKey for security reasons.

@parse-community/android - I don't have any experience with the Android SDK so would someone be able to make a PR for this? - I have made the changes for the iOS SDK in #604.