wg_noise: Drop atomics for kp_can_send.

[jhb]

Summary

Using atomics for only load and store seem pointless. The only ones
that might matter are covered by fences in epoch_*.

[emaste]

Is this patch still relevant?

[jhb]

It's not really critical.

[markj]

I think this is superseded by commit e32e1a160e01a no?

[jhb]

In D35992#1028570, @markj wrote:

I think this is superseded by commit e32e1a160e01a no?

Kind of. This change drops atomics that I think aren't really needed, but they are also fairly harmless. It is true that initially when trying to switch wg(4) to atomic(9) we didn't support bools for atomic(9), but I ended up adding bools to atomic(9) so we could switch without having to remove any. I still think these atomics aren't needed, per se but they are also pretty harmless.