PERF: Improve query speed when looking up direct PMs

PERF: Improve query speed when looking up direct PMs

Follow up to https://github.com/discourse/discourse/commit/5fc5a7f5ae8c3cf1a89c1a685258e9e942de1508

diff --git a/lib/search.rb b/lib/search.rb
index a109f48..9fcd1b7 100644
--- a/lib/search.rb
+++ b/lib/search.rb
@@ -278,7 +278,6 @@ class Search
             ON tau2.topic_id = tau.topic_id
             AND tau2.id != tau.id
             WHERE tau.user_id = :user_id
-            AND tau.topic_id = posts.topic_id
             GROUP BY tau.topic_id
             HAVING COUNT(*) = 1
           )

GitHub sha: f3643176