cgroup: drop cgroup__DEVEL__legacy_files_on_dfl

Now that interfaces for the major three controllers - cpu, memory, io
- are shaping up, there's no reason to have an option to force legacy
files to show up on the unified hierarchy for testing.  Drop it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>