DevTools: remove CSSRule.sourceURL from protocol.json

This patch removes CSSRule.sourceURL from protocol.json as this
information could be retrieved through parent style sheet.

BUG=341506

Review URL: https://codereview.chromium.org/224903019

git-svn-id: svn://svn.chromium.org/blink/trunk@170981 bbb929c8-8fbe-4397-9dbb-9b2b20218538