DevTools: remove unused spinner.gif

This patch removes obsolete spinner.gif and saves us 1.5kb

BUG=

Review URL: https://codereview.chromium.org/163853003

git-svn-id: svn://svn.chromium.org/blink/trunk@167108 bbb929c8-8fbe-4397-9dbb-9b2b20218538