Drop USMetadata support

[adborden]

User Story

In order to reduce the number of extensions we maintain, data.gov developers wants to drop support for USMetadata.

Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using BDD.]

• GIVEN [a contextual precondition]
  [AND optionally another precondition]
  WHEN [a triggering event] happens
  THEN [a verifiable outcome]
  [AND optionally another verifiable outcome]

Background

ckanext-dcat_usmetadata has replaced USMetadata. We need a code audit to identify any USMetadata features still in use by Inventory. The redaction feature is not used by Inventory.

GSA/datagov-ckan-multi#422 (comment)

Security Considerations (required)

[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

Sketch

[Notes or a checklist reflecting our understanding of the selected approach]