syscall: remove //sysnb comment generating Setreuid for linux/arm64

CL 210639 moved the //sysnb for Setreuid from syscall_linux_$GOARCH.go
to syscall_linux.go but forgot to remove the comment from
syscall_linux_arm64.go which leads to Setreuid being generated twice for
linux/arm64. Remove that //sysnb comment to avoid this.

Change-Id: I2c8ad95f786530ca964685b0a4fe463c64764307
Reviewed-on: https://go-review.googlesource.com/c/go/+/350531


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>