Revert "make get_concurrent_links.py cgroup aware"

This reverts commit e88857f9f573da270b5b5fb638c0934fdf960ae1.

Reason for revert: 
cpu_count=1 on bot
e.g.
https://ci.chromium.org/ui/p/chromium/builders/build/linux-build-perf-siso/5051/overview

Original change's description:
> make get_concurrent_links.py cgroup aware
>
> /proc/meminfo and multiprocessing.cpu_count() are not aware of cgroup.
> Use cgroup limit if they are available.
>
> Change-Id: I520987a44a565b03a2b5f78f6e18e86f027acc19
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6406989
> Commit-Queue: Fumitoshi Ukai <ukai@google.com>
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Cr-Commit-Position: refs/heads/main@{#1440718}

Change-Id: Id0d9ad0439c86fd885806589830533397608f38d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6425320
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Cr-Commit-Position: refs/heads/main@{#1441893}