Remove unnecessary Xcode setting for iOS builds.

The line being removed in this CL was originally added to prevent iOS
builds from compiling for multiple archs. Since then, the VALID_ARCHS
setting was added which accomplishes the same thing.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/16831022

git-svn-id: http://src.chromium.org/svn/trunk/src/build@207588 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -4553,7 +4553,6 @@
         ],
       }],
       ['OS=="ios"', {
-        'ARCHS': '$(ARCHS_UNIVERSAL_IPHONE_OS)',
         # Just build armv7, until armv7s is correctly tested.
         'VALID_ARCHS': 'armv7 i386',
         # Target both iPhone and iPad.