Remove flakiness mark for RTCPeerConnection-statsSelector

This is no longer flaky, as verified by flakiness console.
Patch of Sept 9, r157448, has solved the flakiness.

BUG=282620

Review URL: https://codereview.chromium.org/26959005

git-svn-id: svn://svn.chromium.org/blink/trunk@159578 bbb929c8-8fbe-4397-9dbb-9b2b20218538