Remove provisional entry clearing hack in HistoryController.

This was a temporary hack designed to fix a bug in a way that would also
work on the 1750 branch. The cleaner and more correct solution is to never
try to clear HistoryController::m_provisionalEntry and trust goToEntry() to
ensure that no stale provisional entry will be left around after starting
a new provisonal load.

BUG=338480
TEST=WebFrameTest.BackDuringChildFrameReload should still pass

Review URL: https://codereview.chromium.org/143383008

git-svn-id: svn://svn.chromium.org/blink/trunk@166555 bbb929c8-8fbe-4397-9dbb-9b2b20218538