gclient removed syntax validation flag in 2020:
https://crrev.com/c/2083589, so this can safely be removed.

R=aravindvasudev@google.com, gavinmak@google.com

Recipe-Nontrivial-Roll: chrome_release
Recipe-Nontrivial-Roll: build_limited
Recipe-Nontrivial-Roll: build
Change-Id: I2aced6cdb01d571ca361cc57e7c8715c9ca40552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3419278Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>