Remove mousedown preconnect from WebPrescientNetworking

Its implementation is already removed from chromium in r279633.

BUG=386888

Review URL: https://codereview.chromium.org/354073003

git-svn-id: svn://svn.chromium.org/blink/trunk@177040 bbb929c8-8fbe-4397-9dbb-9b2b20218538