internal/poll: remove fallback path in accept

internal/poll: remove fallback path in accept

Support for operating system versions requiring the fallback to
CloseOnExec/SetNonblock was dropped from recent Go versions. The minimum
Linux kernel version is 2.6.32 as of Go 1.18. FreeBSD 10 is no longer
supported as of Go 1.13.

Follows a similar change for net.sysSocket in CL 403634 and
syscall.Socket in CL 422374.

For #45964

Change-Id: I60848415742a1d8204e1fda585462ff35ad6722f
Reviewed-on: https://go-review.googlesource.com/c/go/+/422375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>