Revert search performance regression

We decided it is better to have a less intelligent search (that does not
match search terms in titles) for some people than a bad-performing
search for everyone.

We will revisit the search performance topic in the next release cycle,
possibly with larger changes around indexing.

Refs #1738, #1741, #1764.

src/Discussion/Search/Gambit/FulltextGambit.php

@@ -53,7 +53,7 @@ public function apply(AbstractSearch $search, $bit)
         // discussions that have a relevant title or that contain relevant posts.
         $query
             ->addSelect('posts_ft.most_relevant_post_id')
-            ->leftJoin(
+            ->join(
                 new Expression('('.$subquery->toSql().') '.$grammar->wrapTable('posts_ft')),
                 'posts_ft.discussion_id', '=', 'discussions.id'
             )

tests/integration/api/Controller/ListDiscussionsControllerTest.php

@@ -76,7 +76,7 @@ public function can_search_for_author()
     /**
      * @test
      */
-    public function can_search_for_word_in_title_and_post()
+    public function can_search_for_word_in_post()
     {
         $this->database()->table('posts')->insert([
             ['id' => 2, 'discussion_id' => 2, 'created_at' => Carbon::now()->toDateTimeString(), 'user_id' => 2, 'type' => 'comment', 'content' => '<t><p>not in text</p></t>'],
@@ -98,7 +98,7 @@ public function can_search_for_word_in_title_and_post()
         }, $data['data']);
 
         // Order-independent comparison
-        $this->assertEquals(['2', '3'], $ids, 'IDs do not match', 0.0, 10, true);
+        $this->assertEquals(['3'], $ids, 'IDs do not match', 0.0, 10, true);
     }
 
     /**
@@ -126,7 +126,7 @@ public function ignores_non_word_characters_when_searching()
         }, $data['data']);
 
         // Order-independent comparison
-        $this->assertEquals(['2', '3'], $ids, 'IDs do not match', 0.0, 10, true);
+        $this->assertEquals(['3'], $ids, 'IDs do not match', 0.0, 10, true);
     }
 
     /**