Remove redundant -GN bot suffixes.
The GN in the name only matters for iOS bots now. All the others are GN-only.
This revealed a couple redundant build bots: we had two each of Win x86_64 Debug and Release builders, one with -GN and one not.
I have intentionally not resorted gen_tasks.go to keep the diff looking somewhat sane. I'd be happy to sort here or to follow up with a re-sort CL.
Reviewed-by: Eric Boren <firstname.lastname@example.org>