[Fix] Improvement to Entity.destroy

[mvaligursky]

Fixes #3954

• when the child is removed from the array, clear the parent as well, before calling destroy, to avoid it from trying to remove it from the array again
• use pop instead of shift, which is faster, and we don't really care about the order of children destruction.

[slimbuck]

Could there be logic that depends on still being parented when destroyed?

[mvaligursky]

Could there be logic that depends on still being parented when destroyed?

I would hope not, but this is hard to find out.