DevTools: [CSS] remove setStyleText from protocol.json

This patch removes setStyleText from protocol.json as it is never used
in our front-end.

BUG=120131

Review URL: https://codereview.chromium.org/149323004

git-svn-id: svn://svn.chromium.org/blink/trunk@166503 bbb929c8-8fbe-4397-9dbb-9b2b20218538