Remove unused "date" date format.

All usages have been replaced with "posted" in a previous commit.

	linuxfr.org$ ack -l ':date\b' | grep -v svg$
	app/controllers/admin/accounts_controller.rb
	app/models/account.rb
	app/models/board.rb
	db/migrate/20230506121910_add_last_seen_on_to_accounts.rb

None of the references to ":date" in those three files refer to the custom "date" date format.

Author: Adrien Kunysz

config/initializers/time_formats.rb

@@ -1,6 +1,5 @@
 # encoding: UTF-8
 date_formats = {
-  date:      '%d/%m/%y',
   posted:    ->(t) {I18n.l(t, format: :posted)},
   norloge:   '%H:%M:%S',
   norloge2:  '%Y-%m-%d %H:%M:%S',