Revert "git_cl: add --parallel for yapf"

This reverts commit 8533236d.

Reason for revert:
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2163086/4#message-dcd4603d5914c681d0e1ad498e7bb9b59576a24d

Original change's description:
> git_cl: add --parallel for yapf
> 
> see
> https://github.com/google/yapf#usage
> 
> Bug: 1073708
> Change-Id: I09a327e44bfcf5cf9a59c4e8d470a8886fc9f44b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2163086
> Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=agable@chromium.org,tikuta@chromium.org,ehmaldonado@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1073708
Change-Id: Ib8245dca57a11a386e78089630df61ff773b1789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2168971Reviewed-by: Aaron Gable <agable@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -4921,7 +4921,7 @@ def CMDformat(parser, args):
       if not yapf_style:
         yapf_style = 'pep8'
 
-      cmd = [yapf_tool, '--parallel', '--style', yapf_style, f]
+      cmd = [yapf_tool, '--style', yapf_style, f]
 
       has_formattable_lines = False
       if not opts.full: