DevTools: Remove flaky target-leak.html

Since it is allowed now to hold a reference on dead target, this test isn't actual anymore

BUG=390513
TBR=vsevik

Review URL: https://codereview.chromium.org/403523004

git-svn-id: svn://svn.chromium.org/blink/trunk@178363 bbb929c8-8fbe-4397-9dbb-9b2b20218538