Remove this event which is completely unused. In addition, lots of code
becomes dead once this happens, so remove that as well. It may be
possible that a different behavior is necessary immediately following a
window resize, but the new API will handle this in a different way than
this embedder event -- which complicates how the event loop is spun in
both the API and servoshell.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>