Simplify insertion index calculation for DND reordering.

The original code took a stab at there being some travel resistance
to drags to avoid undesired accidental moves, but aside from doing
this in a naive (and buggy) way, it also turns out to be unnecessary
in further testing: Drag initiation has resistance to it anyway, and
during moves, the lack of resistance makes accidental overshoots
easy to correct for, along with the upside of making DND feel much
more fluid.

BUG:324910