osmo-bts-trx: remove an 'else' branch in _sched_dl_burst()
osmo-bts-trx: remove an 'else' branch in _sched_dl_burst()

When I introduced this branch in change [1], I was under assumption
that PDCHs never have an associated 'struct gsm_lchan'.  This is
incorrect: the association actually happens during PDCH activation,
i.e. when osmo-pcu establishes connection over the PCUIF.

Enforcing attenuation of 0 dB for other channel types makes no sense.

[1] I3dcee6e910ccc61c5c63c728db9ea04327e2fc98

Change-Id: I77a839725f22be1be4a025f579c4c9ccdfb53460

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts