setapi: avoid duplicate setFileAPI() calls for clear log output

Before this change, the cleanup() function would unconditionally call setFileAPI.
That is not incorrect (ultimately a no-op), but results in confusing log output.

PiperOrigin-RevId: 707512218
Change-Id: Id1db4f53a7043f6c6b10500345faf809791bd714
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/637356
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>