Make migrations using remove_index reversible

These issues were detected after upgrading to rubocop-rails >= 2.8.0.

db/migrate/20171002103314_add_poll_shift_task_index.rb

@@ -1,6 +1,6 @@
 class AddPollShiftTaskIndex < ActiveRecord::Migration[4.2]
   def change
-    remove_index "poll_shifts", name: "index_poll_shifts_on_booth_id_and_officer_id"
+    remove_index "poll_shifts", column: [:booth_id, :officer_id]
     add_index :poll_shifts, :task
     add_index :poll_shifts, [:booth_id, :officer_id, :task], unique: true
   end

db/migrate/20171003095936_remove_officer_assigment_composed_index.rb

@@ -1,5 +1,5 @@
 class RemoveOfficerAssigmentComposedIndex < ActiveRecord::Migration[4.2]
   def change
-    remove_index "poll_officer_assignments", name: "index_poll_officer_assignments_on_officer_id_and_date"
+    remove_index "poll_officer_assignments", column: [:officer_id, :date]
   end
 end

db/migrate/20171003212958_add_date_to_poll_shift_composed_index.rb

@@ -1,7 +1,7 @@
 class AddDateToPollShiftComposedIndex < ActiveRecord::Migration[4.2]
   def change
-    remove_index "poll_shifts", name: "index_poll_shifts_on_booth_id_and_officer_id_and_task"
-    remove_index "poll_shifts", name: "index_poll_shifts_on_task"
+    remove_index "poll_shifts", column: [:booth_id, :officer_id, :task]
+    remove_index "poll_shifts", :task
     add_index :poll_shifts, [:booth_id, :officer_id, :date, :task], unique: true
   end
 end

db/migrate/20190321144328_add_config_to_download_settings.rb

@@ -2,7 +2,7 @@ class AddConfigToDownloadSettings < ActiveRecord::Migration[4.2]
   def change
     add_column :download_settings, :config, :integer, default: 0, null: false
 
-    remove_index :download_settings, name: "index_download_settings_on_name_model_and_name_field"
+    remove_index :download_settings, column: [:name_model, :name_field]
     add_index :download_settings, [:name_model, :name_field, :config], unique: true
   end
 end