CSS: remove dead code from InspectorStyleSheet

This patch removes some dead code from InspectorStyleSheet.

TBR=apavlov
BUG=

Review URL: https://codereview.chromium.org/186823002

git-svn-id: svn://svn.chromium.org/blink/trunk@168468 bbb929c8-8fbe-4397-9dbb-9b2b20218538