Removed the ProxyPolicyProvider on iOS.

This policy provider allows overridding Local State using user cloud policy.
This isn't yet used on iOS, so it can be removed until it's needed.

It also ties the global browser state with a single Profile; if multiprofiles
are ever supported on iOS then there must be a solution to which one overrides
Local State. This code shouldn't be enabled until that is clarified.

BUG=275292

Review URL: https://codereview.chromium.org/140613008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248504 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248504