Remove unneeded health status index

We need to check to ensure index "idx_issues_on_health_status_not_null" btree (health_status) WHERE health_status IS NOT NULL is actually used and remove if not.

The migration style guide has a section on removing unneeded indeces, including a Thanos query for checking its use 👉 https://docs.gitlab.com/ee/development/migration_style_guide.html#removing-indexes

See !95580 (comment 1072018431)

The following discussion from !95580 (merged) should be addressed:

• @cablett started a discussion: (+6 comments)

  any_health_status scope

  Here's the SQL query when running the scope any_health_status:

  SELECT "issues".* FROM "issues" WHERE "issues"."health_status" IS NOT NULL

  explain 👉 https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/11600/commands/41220

  We have an index on health status:

  CREATE INDEX idx_issues_on_health_status_not_null ON issues USING btree (health_status) WHERE (health_status IS NOT NULL);

  so that should be OK?

  with_health_status scope

  And when running the scope with_health_status:

  SELECT "issues".* FROM "issues" WHERE "issues"."health_status" = 3

  explain 👉 https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/11600/commands/41221