RAN_Emulation: f_ran_unregister_imsi complement for f_ran_register_imsi

We have a function to register an IMSI but we're missing a function
to unregister it from RAN_Emulation.  This will cause resource leaks
and eventually an overflow of the ImsiTable.  We don't notice this yet
as we don't have long enough running tests in the test suite yet.

Change-Id: I1f5d86c999d4495d661166f98183dfbc48f05f47
1 file changed