Delete nr_DRB_preconfiguration() and replace with existing functions

nr_DRB_preconfiguration() does a "pre-configuration" for the default DRB
for "noS1" mode, i.e., when there is no connection to core network. In
this case, there is no PDU session to be set up, and hence we need to
preconfigure the DRBs.

We replace this function with partially existing functions and a new
function fill_nr_noS1_bearer_config() which returns a bearer
configuration for the noS1 case, also better reflected in the function
name. It has also been moved to asn1_msg.c, away from the actual PDCP
layer.