Don't honour single read per client isc_nm_read() call in the TLSDNS

This reverts commit f17f5e831b that made
following change:

> The TLSDNS transport was not honouring the single read callback for
> TLSDNS client.  It would call the read callbacks repeatedly in case the
> single TLS read would result in multiple DNS messages in the decoded
> buffer.

Turns out that this change broke XoT, so we are reverting the change
until we figure out a proper fix that will keep the design promise and
not break XoT at the same time.

lib/isc/netmgr/tlsdns.c

@@ -1120,10 +1120,6 @@ tls_cycle_input(isc_nmsocket_t *sock) {
 				goto failure;
 			}
 
-			if (atomic_load(&sock->client)) {
-				break;
-			}
-
 			if (pending == 0) {
 				break;
 			}