syscall: remove cloexecSocket fallback path

syscall: remove cloexecSocket fallback path

Support for Linux kernel versions requiring the fallback to CloseOnExec
was dropped from recent Go versions. The minimum Linux kernel version is
2.6.32 as of Go 1.18. The SOCK_CLOEXEC flag for the socket syscall is
supported since kernel version 2.6.27.

Follows a similar change for net.sysSocket in CL 403634.

For #45964

Change-Id: I8b6311f07c4ed7900a9af3ecb2e146c49db08665
Reviewed-on: https://go-review.googlesource.com/c/go/+/422374
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>