6lowpan: nuke net_ieee802154_lowpan() accessor when 6lowpan is disabled

Johannes noted this is not needed, all of the fragment
accessors don't need CONFIG_NET_NS. This goes test compiled with
CONFIG_BT_6LOWPAN=y and a disabled CONFIG_NET_NS.

CC: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: linux-zigbee-devel@lists.sourceforge.net
Cc: David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: NLuis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -379,15 +379,8 @@ net_ieee802154_lowpan(struct net *net)
 {
 	return &net->ieee802154_lowpan;
 }
-#else
-static inline struct netns_ieee802154_lowpan *
-net_ieee802154_lowpan(struct net *net)
-{
-	return NULL;
-}
 #endif
 
-
 /* For callers who don't really care about whether it's IPv4 or IPv6 */
 static inline void rt_genid_bump_all(struct net *net)
 {