Remove unneccesary indices (#4738)

We only look up status_pins by account_id, or account_id and status_id,
never by status_id

db/migrate/20170829215220_remove_status_pins_account_index.rb

@@ -0,0 +1,6 @@
+class RemoveStatusPinsAccountIndex < ActiveRecord::Migration[5.1]
+  def change
+    remove_index :status_pins, :account_id
+    remove_index :status_pins, :status_id
+  end
+end