[media] DocBook/dvbproperty.xml: Remove DTV_MODULATION from ISDB-T

On ISDB-T, each layer can have its own independent modulation,
applied to the carriers that belong to the segments associated
with them. So, there's no sense to define a global modulation
parameter.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>