selftests/run_kselftest.sh: Remove unused $ROOT

Fix the following shellcheck warning:

ROOT appears unused. Verify use (or export if used externally). [SC2034]

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://lore.kernel.org/r/20260320-selftests-fixes-v1-1-79144f76be01@suse.com
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: rbmarliere

tools/testing/selftests/run_kselftest.sh

@@ -21,7 +21,6 @@ else
 fi
 
 . ./kselftest/runner.sh
-ROOT=$PWD
 
 usage()
 {