Depreacate xmlrpc async call

[leogermani]

This method was replaced with a more complete class in #16674

Its usage was replaced in

• Use blog token to add new comment subscriber #16692
• Use blog token to request the jetpack.updateBlog XMLRPC endpoint #16698
• Update/update role use user token #16710

Now it's no longer used and should be deprecated, especially because we don't want to use JETPACK_MASTER_USER token anymore.

Changes proposed in this Pull Request:

• Deprecates Jetpack::xmlrpc_async_call which is no longer used

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Nothing specific to test, just make sure there are no warnings or notices anywhere
• Any thoughts of external plugins/blocks that could be using it?

Proposed changelog entry for your changes:

• Deprecates Jetpack::xmlrpc_async_call

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-16691

Scheduled Jetpack release: September 1, 2020.
Scheduled code freeze: August 25, 2020

Generated by 🚫 dangerJS against 5e342b3

[kbrown9]

I tested this, and everything was fine.

I searched WPDirectory for other plugins using 'xmlrpc_async_call', and it found Jetpack and Slim Jetpack, which implements its own Jetpack::xmlrpc_async_call. So, I think it's fine to deprecate this. If another plugin is using it, the method still works.

I added a few comments below.

[leogermani]

Sorry I missed the code freeze. But accepted Kim's changes and I think this is good to go now!

[kbrown9]

LGTM!

[jeherve]

Cherry-picked to branch-8.9 in 94c5a6a