Revert "[gclient] Enable parallel sync on ARM Mac by default"

This reverts commit 92b780b0bd.

Reason for revert: mac arm experiencing hang

Original change's description:
> [gclient] Enable parallel sync on ARM Mac by default
>
> I realized that gclient sync doesn't use all the cores on ARM Mac.
> This CL changes the default for ARM Mac to use all the available cores.
>
> Change-Id: Id0316e48d2b16a05a397a69bad53172b3d742f4a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5256559
> Commit-Queue: Junji Watanabe <jwata@google.com>
> Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
> Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
> Auto-Submit: Junji Watanabe <jwata@google.com>

Bug: 1427050
Change-Id: I512d1d1ef0034f12fc9054c5234ae688ec281857
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5259766
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

gclient.py

@@ -4003,8 +4003,7 @@ class OptionParser(optparse.OptionParser):
                                        **kwargs)
 
         # Some arm boards have issues with parallel sync.
-        # ARM Mac should be fine.
-        if sys.platform != 'darwin' and platform.machine().startswith('arm'):
+        if platform.machine().startswith('arm'):
             jobs = 1
         else:
             jobs = max(8, gclient_utils.NumLocalCpus())