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");

CHANGELOG.md

@@ -10,6 +10,8 @@
 
 * Removed the option to disable keyboard shortcuts.
 
+* Removed the option to disable tag autocomplete.
+
 ## 2021-01-12
 
 ### Changes