Docs: Remove `minimal-ui` related information

The `minimal-ui` viewport property was introduced in iOS 7.1,
but it will no longer be supported in iOS 8.

Ref: https://developer.apple.com/library/prerelease/ios/releasenotes/General/RN-iOSSDK-8.0/#//apple_ref/doc/uid/TP40014223-CH1-SW83
Close: h5bp/html5-boilerplate#1565