This reverts commit d3cfea0c.

Reason for revert: using self.GSUTIL_CP_SEMAPHORE instead of
global GSUTIL_CP_SEMAPHORE

Original change's description:
> Limit number of concurrent gsutil cp calls
>
> gsutil utilizes as many cores as possible, and will create many threads
> too. In combination with gclient doing the same thing (utilize as many
> cores as possible), this results in creating way too many processes and
> some builders are running out of resources (e.g. can't create threads).
>
> This limits number of gsutil cp commands in git_cache to two.
>
> R=​apolito@google.com, dpranke@google.com
>
> Bug: 1255228, 1249003
> Change-Id: Ifcccaacde6c9e1c60c1da0a0bc5a3e512e158dcc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3219900
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1255228, 1249003
Change-Id: I0fde2477ea620c683b49efaa2a70262bd8e615e2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3221900
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>