[One Discover] Clean-up virtual columns implementation

[tonyghiani]

📓 Summary

With the introduction of the [OneDiscover] Add summary column for Discover and Logs Explorer, we don't need anymore the whole definition/testing for the originally added virtual column, as we discarded this concept in favour of a unified Summary column and custom cell renderers by a contextual experience.

✔ Acceptance criteria

• Remove the definitions of the resource and content virtual columns from the Discover plugin.
• Remove skipped functional tests for the resource and content virtual columns.
• Remove any dead reference to utilities created for the virtual column implementation.
• Update Observability Logs Explorer state to remove resource and content virtual column traces.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)