tc-testing: gitignore, delete plugins directory

when we modfying kernel, commit it to our environment building. we find a error
that is "tools/testing/selftests/tc-testing/plugins" failed: No such file or directory"

we find plugins directory is ignored in
"tools/testing/selftests/tc-testing/.gitignore", but the plugins directory
is need in "tools/testing/selftests/tc-testing/Makefile"

Signed-off-by: liujing <liujing@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20220622121237.5832-1-liujing@cmss.chinamobile.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>