rgw/sal: get_placement_target_names() returns void

rgw/sal: get_placement_target_names() returns void

the function returned an integer error code, but two callers were
incorrectly testing the return value as a boolean

the function just returns placement ids that are in-memory, so none of
the drivers have a failure case; change the return value to void

Fixes: https://tracker.ceph.com/issues/62771
Signed-off-by: Casey Bodley <cbodley@redhat.com>