syscall: remove accept on Linux

syscall: remove accept on Linux

accept is no longer used on Linux since CL 346849 changed Accept to use
accept4 only.

For #45964

Change-Id: I72c13df1457016c4785ec13d356ab89cbca644b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/386415
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>