selftests: forwarding: vxlan_bridge_1q: Remove unused function

Remove `vxlan_ping_test()` which is not used and probably was copied
mistakenly from vxlan_bridge_1d.sh.

This was found while adding an equivalent test for IPv6.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>