tc-testing: gitignore, delete plugins directory

mainline inclusion
from mainline-6.1-rc7
commit 1da9e274
category: bugfix
CVE: NA
Reference:https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/tools/testing/selftests/tc-testing/.gitignore?h=v6.1-rc7&id=1da9e27415bfc54db25c8374331aaf5321185a1d

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

tools/testing/selftests/tc-testing/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 __pycache__/
 *.pyc
-plugins/
 *.xml
 *.tap
 tdc_config_local.py