internal/poll: remove redundant atomics from poll.FD on plan9
After CL 235820 all references to FD.rtimedout and FD.wtimedout
are guarded by mutexes. Therefore they can safely be changed
from type atomic.Bool to bool.
Change-Id: I7ab921d1ad5c7ccc147feb2b0fba58a66b031261
Reviewed-on: https://go-review.googlesource.com/c/go/+/472435 Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>