Remove 'future: True' from the EnableWebBasedSignin policy.

This policy is now supported and removing this attribute makes sure
that it is included in the generated documentation.

BUG=368801

Review URL: https://codereview.chromium.org/380183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282343 0039d316-1c4b-4281-b951-d872f2087c98

components/policy/resources/policy_templates.json

@@ -1017,7 +1017,6 @@
         'dynamic_refresh': False,
         'per_profile': False,
       },
-      'future': True,
       'example_value': False,
       'id': 265,
       'caption': '''Enables the old web-based signin''',