net: remove unused error result from newRawConn

net: remove unused error result from newRawConn

It's currently always nil, and the code gets generally less verbose.

Change-Id: Id4f5f9ac6eac0218dda34b8bd5ef41c633cfaf2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/508396
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>