libweston: fix output reflow on removal

This is regression apparently introduced in
0de859ed, which accidentally swapped the
sign of 'delta_width' in the original call site. If one removes an
output, the remaining outputs on the right are getting moved even
further to the right.

The outputs to the right should be moved to the left instead, to close
the gap left by the removed output.

Reported-by: Tomasz Olszak <olszak.tomasz@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>