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.

app/models/user.rb

@@ -671,7 +671,6 @@ class User < ApplicationRecord
   def initialize_attributes
     self.enable_post_navigation = true
     self.new_post_navigation_layout = true
-    self.enable_sequential_post_navigation = true
     self.enable_auto_complete = true
     self.always_resize_images = true
   end