Remove some unneded code from wizard_controller

Removes some screen exit handler methods from the header - the
implementations are already gone, but these were left by mistake (during
rebasing wip cls).

Removes ShowPreviousScreen - not used anymore.

Removes VLOG(1) and UpdateStatusAreaVisibility from Show*Screen methods.
ShowCurrentScreenSmooth, which is called for all of these methods
already logs the screen that is being shown, and updates the status area
visibilty.

BUG=None

Change-Id: If547b33fc68656e1eda7db8bc9d5128ad26de750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524658
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640954}