cleanup: doh.powerdns.org is not working anymore (#924)

While there, `.../internal/sessionresolver` => `.../sessionresolver`

See https://github.com/ooni/probe/issues/2255

internal/engine/internal/sessionresolver/errwrapper_test.go

@@ -1,24 +0,0 @@
-package sessionresolver
-
-import (
-	"errors"
-	"io"
-	"testing"
-
-	"github.com/google/go-cmp/cmp"
-)
-
-func TestErrWrapper(t *testing.T) {
-	ew := newErrWrapper(io.EOF, "https://dns.quad9.net/dns-query")
-	o := ew.Error()
-	expect := "<https://dns.quad9.net/dns-query> EOF"
-	if diff := cmp.Diff(expect, o); diff != "" {
-		t.Fatal(diff)
-	}
-	if !errors.Is(ew, io.EOF) {
-		t.Fatal("not the sub-error we expected")
-	}
-	if errors.Unwrap(ew) != io.EOF {
-		t.Fatal("unwrap failed")
-	}
-}