Changes

erab_fsm: make erab_release_{cmd,ind}/1 non-blocking

There's no practical benefit(s) in blocking the caller while waiting
for the PFCP Session Deletion Response from the UPF.  The response
does not contain any useful information for the caller.

Make the release related API non-blocking by using gen_statem:cast/2
instead of gen_statem:call/2.  Adjust state functions to not reply.

Change-Id: If519334359c3ef331720cb87bdba2d969601816e
Related: SYS#7318