chore(grouping): Kill grouping levels endpoints (#54073)

The grouping levels and grouping levels new issues endpoints were used to power the Grouping tab on the issue details page, which has since [been removed](#52895). Since they're no longer used anywhere in the codebase (in any of our repos), they can go away.

Author: lobsterkatie

pyproject.toml

@@ -188,7 +188,6 @@ module = [
     "sentry.api.endpoints.group_tagkey_details",
     "sentry.api.endpoints.group_tagkey_values",
     "sentry.api.endpoints.group_tags",
-    "sentry.api.endpoints.grouping_levels",
     "sentry.api.endpoints.index",
     "sentry.api.endpoints.integration_features",
     "sentry.api.endpoints.integrations.index",