[Logs App] Use new getActiveSpace function, abandon getInjectedVar()

[jasonrhodes]

This call here: https://github.com/elastic/kibana/blob/master/x-pack/plugins/infra/public/utils/use_kibana_space_id.ts#L16 is apparently the last thing that is keeping Kibana's legacy spaces plugin alive.

We need to replace that call with the new platform spaces plugin's getActiveSpace() method: https://github.com/elastic/kibana/blob/master/x-pack/plugins/spaces/public/plugin.tsx#L94-L97

Note: the new platform version is an async call, so the useKibanaSpaceId hook will need to be re-thought.

Thanks for bringing this to our attention, @legrego ! <3

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[joshdover]

Just an FYI, this may need to be prioritized for early 7.10. We're working on removing legacy plugin support with a focus on frontend plugins first.