users: remove option to disable autocomplete.

This option was originally added in issue #1747. But only ~350 users
ever disabled autocomplete, only ~120 of these were seen in the last
year, and only 9 new users who signed up in the last year disabled it.

Users wishing to disable autocomplete can use this CSS:

    .ui-autocomplete { display: none !important: }

or this Javascript:

    $("[data-autocomplete]").autocomplete("disable");

app/models/user.rb

@@ -698,7 +698,6 @@ class User < ApplicationRecord
 
   def initialize_attributes
     self.new_post_navigation_layout = true
-    self.enable_auto_complete = true
     self.always_resize_images = true
   end