users: remove 'hide deleted posts' account setting.

This setting automatically added the `-status:deleted` metatag to all searches. This meant deleted
posts were filtered out at the database level, rather than at the html level. This way searches
wouldn't have less-than-full pages.

The cost was that searches were slower, mainly because post counts weren't cached. Normally when you
search for a tag, we can get the post count from the tags table. If the search is actually like
`touhou -status:deleted`, then we don't know the count and we have to calculate it on demand.

This option is being removed because it did the opposite of what people thought it did. People
thought it made deleted posts visible, when actually it made them more hidden.

app/logical/post_query_builder.rb

@@ -70,22 +70,19 @@ class PostQueryBuilder
     COUNT_METATAG_SYNONYMS.flat_map { |str| [str, "#{str}_asc"] } +
     CATEGORY_COUNT_METATAGS.flat_map { |str| [str, "#{str}_asc"] }
 
-  attr_reader :query_string, :current_user, :tag_limit, :safe_mode, :hide_deleted_posts
+  attr_reader :query_string, :current_user, :tag_limit, :safe_mode
   alias_method :safe_mode?, :safe_mode
-  alias_method :hide_deleted_posts?, :hide_deleted_posts
 
   # Initialize a post query.
   # @param query_string [String] the tag search
   # @param current_user [User] the user performing the search
   # @param tag_limit [Integer] the user's tag limit
   # @param safe_mode [Boolean] whether safe mode is enabled. if true, return only rating:s posts.
-  # @param hide_deleted_posts [Boolean] if true, filter out status:deleted posts.
-  def initialize(query_string, current_user = User.anonymous, tag_limit: nil, safe_mode: false, hide_deleted_posts: false)
+  def initialize(query_string, current_user = User.anonymous, tag_limit: nil, safe_mode: false)
     @query_string = query_string
     @current_user = current_user
     @tag_limit = tag_limit
     @safe_mode = safe_mode
-    @hide_deleted_posts = hide_deleted_posts
   end
 
   def metatag_matches(name, value, relation = Post.all, quoted: false)