Revert "gclient: enable syntax validation by default"

This reverts commit 8db72efd.

Reason for revert: maybe caused outage.

Original change's description:
> gclient: enable syntax validation by default
> 
> Please pass --disable-syntax-validation as an escape hatch.
> 
> Bug: 570091
> Change-Id: Iceadc0d2fc54ca68e775c3bc7590f4329ddb4951
> Reviewed-on: https://chromium-review.googlesource.com/509451
> Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> 

TBR=phajdan.jr@chromium.org,dpranke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
Bug: 570091

Change-Id: I5b91653839f09a29c4c633fba91b23260ba96761
Reviewed-on: https://chromium-review.googlesource.com/512342Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

gclient.py

@@ -2001,12 +2001,8 @@ def CMDsync(parser, args):
                     help='DEPRECATED: This is a no-op.')
   parser.add_option('-m', '--manually_grab_svn_rev', action='store_true',
                     help='DEPRECATED: This is a no-op.')
-  # TODO(phajdan.jr): Remove --validate-syntax option once confirmed it works.
-  parser.add_option('--validate-syntax', action='store_true', default=True,
+  parser.add_option('--validate-syntax', action='store_true',
                     help='Validate the .gclient and DEPS syntax')
-  parser.add_option('--disable-syntax-validation', action='store_false',
-                    dest='validate_syntax',
-                    help='Disable validation of .gclient and DEPS syntax.')
   (options, args) = parser.parse_args(args)
   client = GClient.LoadCurrentConfig(options)