syscall: drop fallback to accept in Accept on Linux

syscall: drop fallback to accept in Accept on Linux

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see #45964. The current minimum required version is 2.6.23 and
accept4 was added in 2.6.28, so the fallback to accept in Accept on
Linux can be removed.

For #45964

Change-Id: I78fc4e5b58417bbc540912c9dbf1b1b3db888fea
Reviewed-on: https://go-review.googlesource.com/c/go/+/346849
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>