improve/remove the check on unit height

[Bluestone]

(maps can have arbitrarily high ground)

[beherith]

But in practice they max out at about 4000

[Bluestone]

I think its better if I remove the check (it never existed before & were no problems) and then improve the gameend gadgetry to ignore crashing units?

[Bluestone]

Removed the check, prevent comblast from creating crashing aircraft - fixes #472 in a different way.

(An aircraft could still get blown high into the sky if it was crashing already and happened to be in just the spot when a com exploded beneath it; almost never happens)

Unfortunately its not possible to do a full fix for #472 because Spring.DestroyUnits doesn't affect crashing aircraft and Spring.GetUnitVelocity can't tell early enough if the unit is about to get blown high into the sky or not.