Don't error out when deleting a non-existent pool with transactions

When transactions are enabled, Gitaly returns a not found error when attempting to delete a non-existent object pool. Without transactions, Gitaly returns a successful response. This was missed as the test case testing non-existent pool deletion is actually deleting a pool that exists.

Fix the broken test and adjust behavior with transactions to return the same successful response as without. Arguably returning a not found would be a better interface this will fix Rails pipeline failures with transactions for now.

Closes #5989 (closed)