Drop unnecessary readonly_during_backup setting (#9112)

config/locales/server.en.yml

@@ -1647,7 +1647,6 @@ en:
     discord_secret: "Discord Secret Key"
     discord_trusted_guilds: 'Only allow members of these Discord guilds to login via Discord. Use the numeric ID for the guild. For more information, check the instructions <a href="https://meta.discourse.org/t/configuring-discord-login-for-discourse/127129">here</a>. Leave blank to allow any guild.'
 
-    readonly_mode_during_backup: "Enable read only mode while taking a backup"
     enable_backups: "Allow administrators to create backups of the forum"
     allow_restore: "Allow restore, which can replace ALL site data! Leave false unless you plan to restore a backup"
     maximum_backups: "The maximum amount of backups to keep on disk. Older backups are automatically deleted"

config/site_settings.yml

@@ -1649,8 +1649,6 @@ backups:
   enable_backups:
     default: true
     client: true
-  readonly_mode_during_backup:
-    default: false
   allow_restore:
     default: false
   backup_location:

lib/backup_restore/backuper.rb

@@ -106,7 +106,7 @@ module BackupRestore
         end
 
       @logs = []
-      @readonly_mode_was_enabled = Discourse.readonly_mode? || !SiteSetting.readonly_mode_during_backup
+      @readonly_mode_was_enabled = Discourse.readonly_mode?
     end
 
     def listen_for_shutdown_signal