users: remove option to disable keyboard shortcuts.

Remove the enable_post_navigation option. This option was originally
added to disable the next/prev post navbar beneath posts. It was later
repurposed to disable keyboard shortcuts.

Users who don't want keyboard shortcuts are advised to not press random
buttons on the keyboard like a caveman.

Only ~1200 users disabled this option and only ~600 were seen in the
last year.

app/models/user.rb

@@ -669,7 +669,6 @@ class User < ApplicationRecord
   extend SearchMethods
 
   def initialize_attributes
-    self.enable_post_navigation = true
     self.new_post_navigation_layout = true
     self.enable_auto_complete = true
     self.always_resize_images = true