Remove Push Notifications BETA

[lukabratos]

It's not needed anymore since we've released Beams.

[leesio]

@lukabratos do we know whether people are still using this functionality?

[lukabratos]

There might be some people who're still using this functionality but it's alright because they've probably already configured everything that's needed. For new customers that want this functionality we recommend Beams instead.

Reasons for this PR:

• Links are broken. They're redirecting to Beams and not to the Push Notifications BETA docs.
• We have Beams now which is improved and better service so we would like to reduce confusion by having docs for Push Notifications BETA and Beams. We want to encourage our customers to use new service which is supported better and provides more value for them.
• We have migration guide with detailed instructions how to migrate from Push Notifications BETA to Beams.
• If customers will still have questions they can reach us on community Slack or through support.
• We have already removed this instructions from other libraries: Ruby, PHP, Python, ...

[leesio]

Sorry! I should have clarified. I'm fine with your change. I was just curious/