Remove obsolete path of getSources.

The path for getSources has recently moved to UserMediaClient and this function is obsolete.

BUG=420975

Review URL: https://codereview.chromium.org/631813002

git-svn-id: svn://svn.chromium.org/blink/trunk@183528 bbb929c8-8fbe-4397-9dbb-9b2b20218538