ipvs: avoid netns exit crash on ip_vs_conn_drop_conntrack

commit 8f4e0a18 ("IPVS netns exit causes crash in conntrack")
added second ip_vs_conn_drop_conntrack call instead of just adding
the needed check. As result, the first call still can cause
crash on netns exit. Remove it.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: Simon Horman <horms@verge.net.au>