libmsc/gsm_09_11.c: clean up the local GSM 04.80 API

Since we don't process SS/USSD requests in OsmoMSC anymore, there
are some useless GSM 04.80 functions remained from the past.

In particular, this change does the following:

  - removes both gsm0480_send_{ussd_response|return_error}
    functions because they are not used anymore;

  - changes symbol prefix from 'gsm0480_' to 'msc_', in order to
    avoid possible conflicts with the libosmogsm's GSM 04.80 API;

  - cleans up useless includes;

Change-Id: I2990d8627bce0ce6afb1dcf6b11bb194292380d3
3 files changed