Remove Iglu registry URL

What does this MR do?

Remove Iglu registry URL was added to the application settings on !18449 (merged)

• Applications sending self-describing events or events with custom contexts may use an event definition in some iglu registry.
• But that definition needs to be in the resolver config for snowflow https://ops.gitlab.net/gitlab-com/gitlab-com-infrastructure/blob/master/environments/aws-snowplow/templates/enricher-user-data.sh
• So individual events need not know about which iglu registry their custom event is described in. They only need the schema name

We don't need a configuration for that entity.

Issue #244828 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done