Revert 246117 "gn: Default is_clang to true on iOS."

Revert 246117 "gn: Default is_clang to true on iOS."

This isn't correct for the xcode iOS builder.

> gn: Default is_clang to true on iOS.
>
> Mirrors this condition in build/common.gypi:
>
>       ['OS == "ios"', {
>         # ...
>
>         # Enable clang and host builds when generating with ninja-ios.
>         'conditions': [
>           ['"<(GENERATOR)"=="ninja"', {
>             'clang%': 1,
>             'host_os%': "mac",
>           }]
>         ],
>       }],
>
> BUG=336531
> TBR=brettw@chromium.org
>
> Review URL: https://codereview.chromium.org/143323008

TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/138023008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246144 0039d316-1c4b-4281-b951-d872f2087c98