adapter: Remove the enable_auto_route_introspection_queries LD Flag

[do-not-use-parker-timmerman]

Motivation

Fixes #18457

This PR removes the ENABLE_AUTO_ROUTE_INTROSPECTION_QUERIES LaunchDarkly flag that was introduced to gate #18310. The original feature shipped with v0.49 and this PR should be included in v0.52, so the feature will have had 3 weeks of bake time.

In addition to removing the LD flag, we also remove the note from our docs that indicates users should set their cluster to mz_introspection. With this feature the documented queries get automatically run on this cluster.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • Queries that only depend on system tables, e.g. the SHOW ... commands, will automatically get run on the mz_introspection cluster, which contain in-memory indexes on these tables to speed up the execution.

[jkosh44]

LGTM