Use Status.group instead of Status.distinct in HashQueryService

DISTINCT clause removes duplicated records according to all the selected
attributes. In reality, it can remove duplicated records only looking at
statuses.id, but the clause confuses the query planner and yields
insufficient performance.
The behavior is also problematic if the scope produced by HashQueryService
is used to query columns without id (using pluck method, for example). The
scope is expected to contain unique statuses, but the uniquness will be
evaluated with some arbitrary columns other than id.

GROUP BY clause resolves those problem by explicitly specifying the
column to take into account for the record distinction.

A workaround for the problem of DISTINCT clause in
Api::V1::Timelines::TagController is no longer necessary and removed.

app/controllers/api/v1/timelines/tag_controller.rb

@@ -33,9 +33,7 @@ class Api::V1::Timelines::TagController < Api::BaseController
       )
 
       if truthy_param?(:only_media)
-        # `SELECT DISTINCT id, updated_at` is too slow, so pluck ids at first, and then select id, updated_at with ids.
+        statuses.joins(:media_attachments)
-        status_ids = statuses.joins(:media_attachments).distinct(:id).pluck(:id)
-        statuses.where(id: status_ids)
       else
         statuses
       end

app/services/hashtag_query_service.rb

@@ -8,7 +8,7 @@ class HashtagQueryService < BaseService
     all  = tags_for(params[:all])
     none = tags_for(params[:none])
 
-    Status.distinct
+    Status.group(:id)
           .as_tag_timeline(tags, account, local)
           .tagged_with_all(all)
           .tagged_with_none(none)