Eliminate proto_tree_add_text from packet-gsm_a_bssmap.c

Also remove other_decode_bitfield_value calls.

Change-Id: I056fe74fbd547b690cd5ed60e3e8777df9474704
Reviewed-on: https://code.wireshark.org/review/4468
Reviewed-by: Anders Broman <a.broman58@gmail.com>