[docs] "Performance and scaling" docs page refers to deprecated functionality

[Samuel Zuk]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: ______
- [ ] This is a doc addition request.
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UUID and SQL token storage have been removed, along with the `keystone-manage flush_tokens` command, but the page on tuning Keystone for performance and scalability (https://docs.openstack.org/keystone/pike/admin/identity-performance.html) still mentions a persistent token storage backend and the need to flush expired tokens from said backend.

The entire "Pruning expired tokens from backend storage" should probably be removed on this page, along with mentions of UUID/SQL token storage here and elsewhere in the docs (which may or may not exist)

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-----------------------------------
Release: 12.0.4.dev11 on 2020-06-04 20:20
SHA: a405e4b71d7de31e81a01f07e02f189650eb66fe
Source: https://git.openstack.org/cgit/openstack/keystone/tree/doc/source/admin/identity-performance.rst
URL: https://docs.openstack.org/keystone/pike/admin/identity-performance.html

[Samuel Zuk (szuk)]

I got to the page with this error through a search query, noticed information on it that was false, then clicked the "file bug" link, not seeing that the documentation was for an old release. Whoops. Please ignore/delete/close.