Removed the dedicated downscaling code from scale_surface(), use bilinear interpolation for everything.

It seems to have been intended to be a slower but fancier alternative to the original nearest neighbour scaling. However, NN was replaced by BI long ago and the removed algorithm doesn't seem to have any benefits over BI.

The Pandora code is not affected.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56115 75b3775d-3801-0410-9e12-e2e43121521d