gprs: Remove now unused authenticate flag

This has been obsoleted by the previous commit. Remove it.
diff --git a/openbsc/include/openbsc/gprs_sgsn.h b/openbsc/include/openbsc/gprs_sgsn.h
index 84307a5..48c9cc3 100644
--- a/openbsc/include/openbsc/gprs_sgsn.h
+++ b/openbsc/include/openbsc/gprs_sgsn.h
@@ -275,7 +275,6 @@
 	struct sgsn_mm_ctx	*mm;
 	struct gsm_auth_tuple	auth_triplets[5];
 	int			auth_triplets_updated;
-	int			authenticate;
 };
 
 struct sgsn_config;