selftests: tc-testing: drop '-N' argument from nsPlugin

This argument would bypass the net namespace creation and run the test in
the root namespace, even if nsPlugin was specified.
Drop it as it's the same as commenting out the nsPlugin from a test and adds
additional complexity to the plugin code.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>