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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go