perf: don't preemptively determine the backend in getSeenUsers

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/private/User/Manager.php

@@ -871,13 +871,7 @@ public function getSeenUsers(int $offset = 0, ?int $limit = null): \Iterator {
 			$offset += $batchSize;
 
 			foreach ($userIds as $userId) {
-				foreach ($this->backends as $backend) {
-					if ($backend->userExists($userId)) {
-						$user = new LazyUser($userId, $this, null, $backend);
-						yield $userId => $user;
-						break;
-					}
-				}
+				yield $userId => $this->getExistingUser($userId);
 			}
 		} while (count($userIds) === $batchSize && $limit !== 0);
 	}