Ores: Make 'absheight' flag non-functional

Ores: Make 'absheight' flag non-functional

The 'absheight' flag was added years ago for the floatlands of 'indev'
mapgen (now deleted). The feature mirrored all ore placement around y = 0
to place ores in floatlands.

In MTG we now use dedicated ore registrations for floatlands.

The feature is crude, inflexible, problematic and very rarely used, it
also makes ore vertical range code more complex.
Minetest 0.5 is a good chance to remove the feature.

The flag itself remains to not break flag values.