Remove insertUserStyleSheet function and enum UserStyleSheet.

A UserStyleLevel parameter is no longer used in insertUserStyleSheet()
and insertUserStyleSheet() calls just insertStyleSheet() again.

chromium side: https://codereview.chromium.org/154113008

Review URL: https://codereview.chromium.org/143203006

git-svn-id: svn://svn.chromium.org/blink/trunk@167110 bbb929c8-8fbe-4397-9dbb-9b2b20218538