gnrc_ipv6_nib: disable router advertisements on interface startup

Router Advertisements are enabled automatically for routing nodes
as soon as they obtain a prefix (e.g. via DHCPv6, UHCP, …).

Enabling router advertisements without a prefix to advertise is
wrong and causes no router solicitations to be send on an upstream
interface.

Also from https://www.rfc-editor.org/rfc/rfc4861#section-6.2.1

	Note that AdvSendAdvertisements MUST be FALSE by
	default so that a node will not accidentally start
	acting as a router unless it is explicitly
	configured by system management to send Router
	Advertisements.

sys/include/net/gnrc/ipv6/nib/conf.h

@@ -130,18 +130,11 @@ extern "C" {
 #endif
 
 /**
- * @brief    (de-)activate router advertising at interface start-up
+ * @brief    activate router advertising at interface start-up
  */
 #ifndef CONFIG_GNRC_IPV6_NIB_ADV_ROUTER
-#if CONFIG_GNRC_IPV6_NIB_ROUTER && \
-    (!CONFIG_GNRC_IPV6_NIB_6LR || CONFIG_GNRC_IPV6_NIB_6LBR) && \
-    !(IS_USED(MODULE_DHCPV6_CLIENT_IA_PD) || IS_USED(MODULE_GNRC_UHCPC) || \
-      IS_USED(MODULE_GNRC_IPV6_AUTO_SUBNETS) || IS_USED(MODULE_GNRC_IPV6_STATIC_ADDR))
-#define CONFIG_GNRC_IPV6_NIB_ADV_ROUTER               1
-#else
 #define CONFIG_GNRC_IPV6_NIB_ADV_ROUTER               0
 #endif
-#endif
 
 /**
  * @brief   Include a Route Information Option for subnets