wfe: remove special "multiple certificates" error (#6983)

This was introduced early in Boulder development when we had the concept
of a "short serial" (monotonically increasing) which would be prepended
to random bytes to form the full serial. We wanted to specially report
the case that there were duplicates of a given short serial since it
meant a problem with our monotonicity.

We've long since abandoned that idea, and also this code can't be
exercised because sa.SelectCertificate does a LIMIT 1 anyhow.

Author: jsha

wfe2/wfe.go

@@ -1583,9 +1583,7 @@ func (wfe *WebFrontEndImpl) Certificate(ctx context.Context, logEvent *web.Reque
 
 	cert, err := wfe.sa.GetCertificate(ctx, &sapb.Serial{Serial: serial})
 	if err != nil {
-		if strings.HasPrefix(err.Error(), "gorp: multiple rows returned") {
-			wfe.sendError(response, logEvent, probs.Conflict("Multiple certificates with same serial"), nil)
-		} else if errors.Is(err, berrors.NotFound) {
+		if errors.Is(err, berrors.NotFound) {
 			wfe.sendError(response, logEvent, probs.NotFound("Certificate not found"), nil)
 		} else {
 			wfe.sendError(response, logEvent, web.ProblemDetailsForError(err, "Failed to retrieve certificate"), err)