Removes an unreferenced function. Also adds the ability for samples to
respond to key up and down events. Only implemented on Windows.

Bug: angleproject:2830
Change-Id: I44c9f93f0ad07b92923ffc0efa580f97d9b98693
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1908448Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>