CS: Merge Instance Drop Modes

A user recently reported to me that the Drop To Collision keybind in the CS was no longer working. After looking over the code, I finally understood there are four different drop functions, three of which don't need to exist, and the more obviously useful two of those aren't even bound.

There's:

1. Drop to Collision
2. Drop to Terrain
3. Drop to Collision Separately
4. Drop to Terrain Separately.

There are a few problems here, namely that I can't fathom a situation in which you'd want to drop items to the colllision point of only the very first one. This results in weird situations, such as wanting to drop a handful of items on the ground and some on a table, but half of them end up floating at the table's height. or, you use the bind, and don't realize that now a bunch of the objects you selected are now subterranean.

Anyway, I propose merging the binds together into a single function that is equivalent to the current behavior of Drop to Collision Separately (which also includes terrain because of course it does).