Revert "[ios] Enable New Build System in generated Xcode project"

This reverts commit 15168c9b.

Reason for revert: Xcode display errors when running EG2 tests (see
https://crbug.com/1103230).

Original change's description:
> [ios] Enable New Build System in generated Xcode project
>
> Since gn now generates a project that wraps the invocation
> of ninja in a script converting relative paths to absolute
> paths, there is nothing blocking the use of the New Build
> System.
>
> Bug: 852522
> Change-Id: I9b4ca343dc9434efb64a3866ca88d6b7c4065760
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276281
> Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#785343}

TBR=sdefresne@chromium.org,olivierrobin@chromium.org

Bug: 852522, 1103230
Change-Id: I5f8825ebd8b08bd89f0013ac278295632a83e3e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2287548
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786319}

ios/build/tools/setup-gn.py

@@ -195,7 +195,6 @@ class GnGenerator(object):
     if generate_xcode_project:
       gn_command.append('--ide=xcode')
       gn_command.append('--root-target=gn_all')
-      gn_command.append('--xcode-build-system=new')
       gn_command.append('--ninja-executable=autoninja')
       if self._settings.has_section('filters'):
         target_filters = self._settings.values('filters')