RRC PDU Session Release: DRBs and PDU session teardown in NGAP PDU Session Release Response callback

We need to keep PDU sessions and DRBs in list until the NGAP PDU Session Release Response is sent.

* Introduced find_drb() and remove_drbs_by_pdu_session() helpers to locate
  and clean up DRBs linked to a specific PDU session
* Targeted per-session DRB removal in rrc_gNB_send_NGAP_PDUSESSION_RELEASE_RESPONSE()
* Ensures DRBs for unaffected PDU sessions remain intact
* Remove unused PDU_SESSION_STATUS_RELEASED status: keeping this status is no longer relevant
  since the PDU sessions are effectively removed from the RRC lists

Closes #867

Also, refactor rrc_gNB_process_NGAP_PDUSESSION_RELEASE_COMMAND

3GPP TS 38.413 8.2.2 PDU Session Resource Release says:

> upon reception the NG-RAN node shall execute the release of the requested PDU sessions.
> For each PDU session to be released the NG-RAN node shall release the corresponding
> resources over Uu and over NG, if any.

Therefore, any PDU session in the pdusession_release_params list that is
also present in the RRC list can be set to release, regardless of its status.

The function signature was also updated along with other minor changes.