fix(invitations): remove spurious index from schema.rb

The batch_and_invitation index was manually added to schema.rb without
a corresponding migration. Running rails db:drop db:create db:migrate
confirmed this index is not created by any migration.

Author: mroderick

db/schema.rb

@@ -297,7 +297,6 @@
     t.datetime "processed_at"
     t.string "status", default: "success", null: false
     t.datetime "updated_at", null: false
-    t.index ["invitation_log_id", "invitation_type", "invitation_id"], name: "index_invitation_log_entries_on_batch_and_invitation"
     t.index ["invitation_log_id", "status"], name: "index_invitation_log_entries_on_invitation_log_id_and_status"
     t.index ["invitation_log_id"], name: "index_invitation_log_entries_on_invitation_log_id"
     t.index ["invitation_type", "invitation_id"], name: "index_invitation_log_entries_on_invitation"