task/cgroup: delete job step memcg instead of using force_empty

The current task/cgroup memory code writes to force_empty at job step
completion and then waits for the release agent to be triggered to
remove the memcg. However, force_empty only causes clean cache pages
to be dropped from the memcg and does not actually move charges to
the parent [1].

This has two unfortunate side-effects. First, pages that can't be
dropped by force_empty are in-use and could stay that way indefinitely
(e.g. system library that is in-use until just after force_empty
completes). Thus, the step memcg never becomes 'empty' and the release
agent is not activated. Second, cached pages that can be freed are
likely associated with the job itself, and those files and libraries
will have to be paged in again for subsequent job steps.

In contrast, calling rmdir(2) on a memcg with no active tasks
causes *all* current charges to move to parent, which is really what
we want in this case. This allows cached libraries and binaries to
stay resident and be associated with the job, and also ensures that
the step memcg is removed immediately as the job step ends.

Thus, this patch replaces the write to force_empty with a call
to xcgroup_delete() on the step memcg, which in turn removes
the memcg with rmdir(2).

The functionality of this patch depends on the previous fix that
uses xcgroup_move_process() to move slurmstepd to the root memcg.
Otherwise, there will be leftover slurmstepd threads in the job
step memcg, and the rmdir will fail with EBUSY.

 [1] Sec 4.3: http://www.kernel.org/doc/Documentation/cgroups/memory.txt