desktop-shell: Don't reposition windows on every resize

Remove any attempt to reposition windows when handling an output resize.

This had some side-effects on x11 and wayland backends that keep moving
windows at random positions when manually the output.

A more smarter approach would be to move them only if they're actually
affected by a resize -- like no longer in the output's viewing area, but
that remains for another day.

Fixes: #793

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reported-by: Philipp Zabel <p.zabel@pengutronix.de>