osmo-bts-trx: bts_model_l1sap_down(): remove chan_nr patching
osmo-bts-trx: bts_model_l1sap_down(): remove chan_nr patching

Thanks to [1], there is no need to patch chan_nr for PDCH anymore.
The upper layers are now expected to use the correct values.

[1] I01680140c7201bf5284b278bceaea8ae01c122b2

Change-Id: I26eb2081eddb4ce900a7867897bc69c3f77dbd73
Related: OS#5238

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