net: xfrm: remove duplicate output_mark

Running Google's Android Tests failed due to the output_mark
attribute appearing twice in netlink messages. This change removes
the duplicate nla_put_u32() call that seems to have entered the
code during a merge conflict.

Change-Id: I8c87c8f4ae7ca91c11853f9f149d18c339fd8377
CRs-Fixed: 2212156
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>