Get rid of method setProperty(): never used in the whole code

The method setProperty(CSSPropertyID, CSSPropertyID identifier, bool important = false) is not used anywhere in the code, so removing it.

Review URL: https://codereview.chromium.org/547333002

git-svn-id: svn://svn.chromium.org/blink/trunk@181606 bbb929c8-8fbe-4397-9dbb-9b2b20218538