cmd/go/internal/get: make the implementation of charsetReader match its comment

The doc comment for charsetReader claims that it supports UTF-8,
but in practice it does not: instead, it is never invoked for UTF-8.

We could update the comment to clarify that fact, but it seems simpler
to change the implementation to match the comment.

Change-Id: I39b11395ccef3feff96480b9294e8f2a232728dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/189777


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>