kunit: tool: drop unused KernelDirectoryPath var

Commit be886ba9 ("kunit: run kunit_tool from any directory")
introduced this variable, but it was unused even in that commit.

Since it's still unused now and callers can instead use
get_kernel_root_path(), delete this var.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>