[Logs / Metrics UI] [NP followup] Remove the legacy plugin

[Kerry350]

This ticket can now be implemented, as Saved Objects Support is in NP

Right now a legacy version of the infra plugin still exists (x-pack/legacy/plugins/infra/index.ts), this is only to facilitate savedObjectMappings whilst they're unavailable in the new platform. Once savedObjectMappings support is fully migrated to NP, we can delete the legacy folder in it's entirety.

The docs here can be used to guide the migration.

[elasticmachine]

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

[weltenwort]

ℹ️ According to #57430, registering the saved objects via the new platform APIs should be possible now.