Revert "gunit/test-015: Populate subtree_control for leaf cgroup"

Revert "gunit/test-015: Populate subtree_control for leaf cgroup"

This reverts commit 328ec5e8e4f00790cb91f81ff53f2cdd42d0106f.

Revert the logic to rely on the leaf cgroup node's subtree_control file
to examine the enabled controllers, this breaks the cgroup v2's no
internal process constraint and fallback to the original idea of relying
upon on leaf cgroup nodes, parent subtree_control to examine the enabled
controllers.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>