promisor-remote: remove a return value

promisor-remote: remove a return value

No caller of promisor_remote_get_direct() is checking its return value,
so remove it.

Not checking the return value means that the user would not know
whether the failure of reading an object is due to the promisor remote
not supplying the object or because of local repository corruption, but
this will be fixed in a subsequent patch.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>