DevTools: Get rid of "Matched CSS rules" label in styles sidebar.

This is done as part of the effort to improve styles sidebar UX and reduce vertical space waste.

BUG=267759

Review URL: https://chromiumcodereview.appspot.com/21976004

git-svn-id: svn://svn.chromium.org/blink/trunk@155483 bbb929c8-8fbe-4397-9dbb-9b2b20218538