remove willRequestAfterPreconnect

These were added to measure effect of mousedown preconnecting, but the
experiment is no longer active and no caller of these functions exist.

BUG=386888

Review URL: https://codereview.chromium.org/397493003

git-svn-id: svn://svn.chromium.org/blink/trunk@178134 bbb929c8-8fbe-4397-9dbb-9b2b20218538