cleanup(netx): remove unused ChainResolver (#657)

This is another cleanup point mentioned by https://github.com/ooni/probe/issues/1956.

While there, fix a bunch of comments in jafar that were incorrectly
referring to the netx package name.

internal/engine/netx/resolver/chain.go

@@ -1,51 +0,0 @@
-package resolver
-
-import (
-	"context"
-
-	"github.com/ooni/probe-cli/v3/internal/model"
-)
-
-// ChainResolver is a chain resolver. The primary resolver is used first and, if that
-// fails, we then attempt with the secondary resolver.
-type ChainResolver struct {
-	Primary   model.Resolver
-	Secondary model.Resolver
-}
-
-// LookupHost implements Resolver.LookupHost
-func (c ChainResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
-	addrs, err := c.Primary.LookupHost(ctx, hostname)
-	if err != nil {
-		addrs, err = c.Secondary.LookupHost(ctx, hostname)
-	}
-	return addrs, err
-}
-
-// Network implements Resolver.Network
-func (c ChainResolver) Network() string {
-	return "chain"
-}
-
-// Address implements Resolver.Address
-func (c ChainResolver) Address() string {
-	return ""
-}
-
-// CloseIdleConnections implements Resolver.CloseIdleConnections.
-func (c ChainResolver) CloseIdleConnections() {
-	c.Primary.CloseIdleConnections()
-	c.Secondary.CloseIdleConnections()
-}
-
-// LookupHTTPS implements Resolver.LookupHTTPS
-func (c ChainResolver) LookupHTTPS(
-	ctx context.Context, domain string) (*model.HTTPSSvc, error) {
-	https, err := c.Primary.LookupHTTPS(ctx, domain)
-	if err != nil {
-		https, err = c.Secondary.LookupHTTPS(ctx, domain)
-	}
-	return https, err
-}
-
-var _ model.Resolver = ChainResolver{}

internal/engine/netx/resolver/chain_test.go

@@ -1,29 +0,0 @@
-package resolver_test
-
-import (
-	"context"
-	"testing"
-
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
-	"github.com/ooni/probe-cli/v3/internal/netxlite"
-)
-
-func TestChainLookupHost(t *testing.T) {
-	r := resolver.ChainResolver{
-		Primary:   resolver.NewFakeResolverThatFails(),
-		Secondary: &netxlite.ResolverSystem{},
-	}
-	if r.Address() != "" {
-		t.Fatal("invalid address")
-	}
-	if r.Network() != "chain" {
-		t.Fatal("invalid network")
-	}
-	addrs, err := r.LookupHost(context.Background(), "www.google.com")
-	if err != nil {
-		t.Fatal(err)
-	}
-	if addrs == nil {
-		t.Fatal("expect non nil return value here")
-	}
-}