The use of security_mode_completed in the PDCP entity was a hack.

Plus it was not working at all with the deregistration request
when nea2 is used, sent when doing ctrl+c in the nrUE.

So let's remove it.

And let's handle activation of integrity and ciphering more in
accordance with what 38.331 says. SecurityModeComplete has to
be sent integrity protected, but not ciphered. Only after should
the ciphering be activated. (See 38.331 5.3.4.3.)