Prior to this commit, after RA failure, the DU would send a UE context
release complete, without actually requesting one. This could lead to
inconsistent state between DU and CU, because the CU might just discard
this (unrequested) message, and not actually trigger a release [1].

Thus, refactor the release request into a function, and use it to
request release of a UE for which RA has not been completed. If the CU
does not know the UE, release the UE immediately.

[1] One possibility would be that during reestablishment, the CU would
request to look up an old DU UE ID when it has been released, resulting
in

    Assertion (oldUE) failed!
    In dl_rrc_message_transfer() /home/oaicicd/robert/openairinterface5g/openair2/LAYER2/NR_MAC_gNB/mac_rrc_dl_handler.c:860
    CU claims we should know UE bc7f, but we don't