Don't trigger `GCN_PRELIM_SENT` on gcn-classic

GCN-Classic is the least stable of the three alert pathways, and while we haven't had issues on production that I've caught, this did cause issues on playground where we couldn't send voevents to the broker for several days and as a result saw strange issues with the DQR_REQUEST label.

Ideally, we'd rework this bit of the code to add the label if any of the three pathways succeeded. Maybe the easiest thing to do would be to add the label after each of the three, since I believe we have other examples in the code of relying on gracedb to just reject the attempt to add a label that's already there.

Relevant mattermost discussion here.