system.conf.sample: Remove reference to single frequency signalling.

Single frequency signalling has not been supported in dahdi_cfg since the switch
to DAHDI. Before DAHDI, in the Zaptel days, it was only suported in the
ztcfg_dude utility which wasn't mainlined. So this removes references to those
signalling modes unless support is added back into the driver.

Internal-Issue-ID: DAHLIN-335
Signed-off-by: Shaun Ruffell <sruffell@digium.com>