users: remove option to disable search navbar.

Remove the enable_sequential_post_navigation option. This option was
used to disable the next/previous post navbar below posts.

This option was originally added in issue #674 because of people
complaining about the navbar when it was originally added. Also there
were complaints about URLs being uglier because of search params in the
URL (e.g. /posts/1234?q=touhou). There were also various minor bugs with
it at the time, such as keyboard shortcuts not working correctly, or the
page not remembering your search after a tag edit.

These complaints are irrelevant nowadays because a) people are used to
the navbar by now (and more often complain about it *not* being there
for order:score searches), b) post URLs always contain the search now,
this option hasn't disabled that for years, and c) the initial bugs with
it were fixed years ago.

Only ~1000 users disabled this option and only ~600 were seen in the last year.

Users still wishing to hide the search navbar can use custom CSS instead.

CHANGELOG.md

@@ -6,6 +6,8 @@
   the navbar beneath posts that lets you move to the next or previous post in a
   tag search. Previously this was only available to logged-in users.
 
+* Removed the option to disable the next/previous post navbar.
+
 ## 2021-01-12
 
 ### Changes