gtp: drop unnecessary call to skb_dst_drop

The call to skb_dst_drop() is already done as part of udp_tunnel_xmit().

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Acked-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>