Remove some testing of getPropertyCSSValue in fast/css/getComputedStyle

As getPropertyCSSValue is no longer web-exposed we should stop testing
it so we can delete the interface.

BUG=428595

Review URL: https://codereview.chromium.org/703793002

git-svn-id: svn://svn.chromium.org/blink/trunk@184888 bbb929c8-8fbe-4397-9dbb-9b2b20218538