gprs_gmm: Remove unused variable

Fixes following compilation warning:
osmo-sgsn/src/gprs/gprs_gmm.c: In function ‘gsm48_rx_gmm_service_req’:
osmo-sgsn/src/gprs/gprs_gmm.c:1786:10: warning: variable ‘ciph_seq_nr’ set but not used [-Wunused-but-set-variable]
  uint8_t ciph_seq_nr, service_type, mi_len, mi_type;
          ^~~~~~~~~~~

Change-Id: If0819026a99f45df96ee6de26a71c16128e79cb8