Remove plugin from documentation

The current users guide has [this plugin](https://github.com/dany74q/keyrings.osx_keychain_keys) listed as providing third-party support for the MacOS keychain. However, based on my latest ad-hoc testing, this plugin is no longer compatible with the latest version of `keyring`. See my bug report [here](https://github.com/dany74q/keyrings.osx_keychain_keys/issues/8).

Given that this plugin hasn't been updated in nearly 5 years, I suspect it won't be getting a fix any time soon. As such you may want to update your users guide to remove the link to it to avoid confusion.

Find the current users guide and locate the link to the dany74q/keyrings.osx_keychain_keys plugin. Remove that outdated third-party support link, then search the guide to confirm the plugin is no longer listed.

Written by the indexing model from the issue text.