Remove testing of getPropertyCSSValue in shorthand getComputedStyle tests

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/707723003

git-svn-id: svn://svn.chromium.org/blink/trunk@184925 bbb929c8-8fbe-4397-9dbb-9b2b20218538