[ios] Fix //ios/build/tools/setup-gn.py after roll of gn

The CL https://gn-review.googlesource.com/c/gn/+/10140 was brought
by the roll of gn. This CL causes the --root-target to have two
conflicting meaning.

Remove the parameter from //ios/build/tools/setup-gn.py to allow
the script to successfully generate an Xcode project. The drawback
is that more target than necessary may be built when building "All"
in Xcode.

Bug: 1133101
Change-Id: I97073fffbea37c0f61691b9599ff0982e4c027c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517585
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823939}

ios/build/tools/setup-gn.py

@@ -203,7 +203,6 @@ class GnGenerator(object):
     gn_command = [ gn_path, '--root=%s' % os.path.realpath(src_path), '-q' ]
     if generate_xcode_project:
       gn_command.append('--ide=xcode')
-      gn_command.append('--root-target=gn_all')
       gn_command.append('--ninja-executable=autoninja')
       if self._settings.has_section('filters'):
         target_filters = self._settings.values('filters')