fix(portal): Remove redundant index on actor_group_memberships (#9063)

Why:

* It was pointed out that the way Postgresql does compound indexes there
is no need to have an individual index on the first column of the
compound index. This commit removes the redundant index on the
`actor_id` for the `actor_group_membership` table.

elixir/apps/domain/priv/repo/migrations/20250509035642_remove_redundant_index_on_actor_group_memberships.exs

@@ -0,0 +1,12 @@
+defmodule Domain.Repo.Migrations.RemoveRedundantIndexOnActorGroupMemberships do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def up do
+    execute("DROP INDEX CONCURRENTLY IF EXISTS actor_group_memberships_actor_id_index")
+  end
+
+  def down do
+    create(index("actor_group_memberships", [:actor_id], concurrently: true))
+  end
+end