shell: do not advertise reactively in 6ctx command

After thinking about it a bit, I decided to remove the reactive
advertisement of context information all together in the shell command.
RFC 6775 does not explicitly forbids, but it discourages multicast RAs,
which would be the alternative.

sys/shell/commands/sc_gnrc_6ctx.c

@@ -57,11 +57,6 @@ int _gnrc_6ctx_list(void)
     return 0;
 }
 
-static void _adv_ctx(void)
-{
-    /* TODO: update NIB, send router advertisement */
-}
-
 int _gnrc_6ctx_add(char *cmd_str, char *ctx_str, char *prefix_str, char *ltime_str)
 {
     ipv6_addr_t prefix;
@@ -99,8 +94,6 @@ int _gnrc_6ctx_add(char *cmd_str, char *ctx_str, char *prefix_str, char *ltime_s
         puts("ERROR: can not add context");
         return 1;
     }
-    /* advertise new/updated context */
-    _adv_ctx();
     return 0;
 }