remove assertion UE DCI handling

Sometimes the UE complains that

Assertion (ra_header==0) failed! In conv_rballoc() /home/ubuntu/tmp/openair1/PHY/LTE_TRANSPORT/dci_tools_common.c:360 resource type 1 not supported for N_RB_DL=50

This is probably due to a badly detected DCI and does not need an assertion. A simple warning will do.