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

+class RemoveStatusPinsAccountIndex < ActiveRecord::Migration[5.1]
+  def change
+    remove_index :status_pins, :account_id
+    remove_index :status_pins, :status_id
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170824103029) do
+ActiveRecord::Schema.define(version: 20170829215220) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -288,8 +288,6 @@ ActiveRecord::Schema.define(version: 20170824103029) do
     t.datetime "created_at", default: -> { "now()" }, null: false
     t.datetime "updated_at", default: -> { "now()" }, null: false
     t.index ["account_id", "status_id"], name: "index_status_pins_on_account_id_and_status_id", unique: true
-    t.index ["account_id"], name: "index_status_pins_on_account_id"
-    t.index ["status_id"], name: "index_status_pins_on_status_id"
   end
 
   create_table "statuses", force: :cascade do |t|