selftests: test_vxlan_under_vrf: mute unnecessary error message

selftests: test_vxlan_under_vrf: mute unnecessary error message

The cleanup function in this script that tries to delete hv-1 / hv-2
vm-1 / vm-2 netns will generate some uncessary error messages:

Cannot remove namespace file "/run/netns/hv-2": No such file or directory
Cannot remove namespace file "/run/netns/vm-1": No such file or directory
Cannot remove namespace file "/run/netns/vm-2": No such file or directory

Redirect it to /dev/null like other commands in the cleanup function
to reduce confusion.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Link: https://lore.kernel.org/r/20201211042420.16411-1-po-hsu.lin@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>