Various code cleanup (#1782)

* Remove duplicate in_chosen_languages definition

* Use `DEFAULT_FIELDS_SIZE` instead of `MAX_FIELDS` to reduce code differences with upstream

* Remove duplicate annotation

* Fix incorrect cross-flavor imports

* Remove deprecated `hide_network` setting (replaced by account column)

* Remove unused KeywordMutesHelper

* Remove trailing spaces

* Remove commit_hash from InstancePresenter, as it has been unused since mid-2017

app/models/mute.rb

@@ -6,7 +6,6 @@
 #  id                 :bigint(8)        not null, primary key
 #  created_at         :datetime         not null
 #  updated_at         :datetime         not null
-#  hide_notifications :boolean          default(TRUE), not null
 #  account_id         :bigint(8)        not null
 #  target_account_id  :bigint(8)        not null
 #  hide_notifications :boolean          default(TRUE), not null