chan_dahdi.conf.sample: Avoid warnings with default configs.

chan_dahdi.conf.sample: Avoid warnings with default configs.

callgroup and pickupgroup may only be specified for FXO-signaled channels;
however, the chan_dahdi sample config had these options uncommented in
the [channels] section, thus applying these settings to all channels,
resulting in warnings. Comment these out so there are no warnings with
an unmodified sample config.

Resolves: #1552