Revert "git-cl: Execute clang-format-diff.py using vpython."

This reverts commit beec6607.

Reason for revert:
OSError: Execution failed with error: [Errno 2] No such file or directory: 'vpython'. 

Original change's description:
> git-cl: Execute clang-format-diff.py using vpython.
> 
> When running git-cl using vpython3, sys.executable is python3.
> Execute clang-format-diff.py using vpython instead, since it has
> not been migrated already.
> 
> Bug: 1066187
> Change-Id: I31556acb50ff4256706a313c1393eddad9f903f3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2130926
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,apolito@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com

Change-Id: I555251d8dbb1582d7f03702b7910d70a11b75855
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1066187
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2130785Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -4758,7 +4758,7 @@ def _RunClangFormatDiff(opts, clang_diff_files, top_dir, upstream_commit):
     except clang_format.NotFoundError as e:
       DieWithError(e)
 
-    cmd = ['vpython', script, '-p0']
+    cmd = [sys.executable, script, '-p0']
     if not opts.dry_run and not opts.diff:
       cmd.append('-i')