Change not to include NSSAI IE in cleartext IEs in Registration Request.

[s5uishida]

When using srsue by adding [slicing] section to ue.conf as NR-UE, change not to include NSSAI IE in Plain NAS 5GS Message of InitialUEMessage according to the following specifications.

3GPP TS 24.501 (v17.8.0) - 4.4.6 Protection of initial NAS signalling messages

When the initial NAS message is a REGISTRATION REQUEST message, the cleartext IEs are:

- Extended protocol discriminator;
- Security header type;
- Spare half octet;
- Registration request message identity;
- 5GS registration type;
- ngKSI;
- 5GS mobile identity;
- UE security capability;
- Additional GUTI;
- UE status;
- EPS NAS message container;
- NID; and
- PLMN with disaster condition.

This will succeed registration request to Open5GS.