Fix APN printing

Previously first character of APN was omitted. Fix this by getting rid
of errorneous wrapper function and using osmo_apn_to_str() directly.

The breaking change was introduced in
I7315ffcbed8a54cca2056f313bb7783ad82d0ee9.

Change-Id: Ie13b6cd0066e0370e2adf48d12a363c2e405eaf2
Related: SYS#3610