Use default argument for server selection (which defaults to primary)

Author: alcaeus

tests/TransactionTest.php

@@ -425,6 +425,6 @@ public function testRollBackWithoutSession(): void
 
     private function getPrimaryServerType(): int
     {
-        return DB::getMongoClient()->getManager()->selectServer(new ReadPreference(ReadPreference::RP_PRIMARY))->getType();
+        return DB::getMongoClient()->getManager()->selectServer()->getType();
     }
 }