lib: remove prio_heap

The prio_heap code is unused since commit 889ed9ce ("cgroup: remove
css_scan_tasks()").  It should be compiled out to shrink the binary
kernel size which can be done via introducing CONFIG_PRIO_HEAD or by
removing the code.

We can simply recover the code from git when needed, so it would be
better to remove it IMO.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Francesco Fusco <ffusco@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: George Spelvin <linux@horizon.com>
Cc: Mark Salter <msalter@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>