cmd/go/internal/get: make the implementation of charsetReader match its comment
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

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