Remove deprecated WebKit gradient syntax

Safari 6.0 (OSX) is reporting cssgradients support despite not supporting the standard syntax (http://dev.w3.org/csswg/css-images-3/#gradients).

As of January 2011, the WebKit team moved to support the new syntax (see: https://www.webkit.org/blog/1424/css3-gradients/ ). The old syntax should be considered deprecated and browsers which lack support for the newer syntax should fail the "cssgradients" test.