Remove DocSearch meta tags

[thibaudcolas]

Cleaning up so our Algolia DocSearch integration for Wagtail itself is easier to manage in the future: we can have the meta tags defined on wagtail/wagtail, like all other aspects of the integration. And:

• docsearch:name is useless (it’ll be the same for all builds of the docs)
• docsearch:package_type is empty
• docsearch:release is less apporpriate than docsearch:version.

Even docsearch:version isn’t well set up currently. Using the version variable means both the stable and vX.Y variants of the documentation would be indexed with the same version. We need to switch to the Read the Docs current_version instead (see wagtail/wagtail#10207).