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/views/users/edit.html.erb

@@ -62,7 +62,6 @@
         <%= f.input :new_post_navigation_layout, :as => :select, :label => "Navigation bar position", :include_blank => false, :collection => [["Below", "true"], ["Above", "false"]], :hint => "When browsing pools or posts, place navigation links above or below the image" %>
         <%= f.input :hide_deleted_posts, :as => :select, :label => "Deleted post filter", :hint => "Remove deleted posts from search results", :include_blank => false, :collection => [["Yes", "true"], ["No", "false"]] %>
         <%= f.input :show_deleted_children, :as => :select, :label => "Show deleted children", :hint => "Show thumbnail borders on parent posts even if the children are deleted", :include_blank => false, :collection => [["Yes", "true"], ["No", "false"]] %>
-        <%= f.input :enable_auto_complete, :as => :select, :hint => "Enable tag autocomplete in the search box", :collection => [["Yes", "true"], ["No", "false"]], :include_blank => false %>
         <%= f.input :disable_categorized_saved_searches, :hint => "Don't show dialog box when creating a new saved search", :as => :select, :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false %>
         <%= f.input :enable_private_favorites, :as => :select, :hint => "Make your favorites private", :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false %>
         <%= f.input :disable_tagged_filenames, :as => :select, :hint => "Don't include tags in image filenames", :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false %>