Fix: Remove username/password elements

Fix: Remove username/password elements

The reason for this is twofold.  First the current code actually leaks
memory since it uses g_strdup on username/password and never frees it
anywhere.  Second the username/password command can just be sent inside
activate_primary, no reason to do it in the callback.

The fix removes the username / password structure members and moves the
AT*EIAAUW handling to activate_primary.  This is almost exactly like the
username / password / context definition for MBM modems.