cleanup: remove ConnID, DialID, TransactionID (#395)

We are not using them anymore. The only nettest still using the
legacy netx implementation is tor, for which setting these fields
is useless, because it performs each measurement into a separate
goroutine. Hence, let us start removing this part of the legacy
netx codebase, which is hampering progress in other areas.

Occurred to me while doing testing for the recent changes in
error mapping (https://github.com/ooni/probe/issues/1505).

internal/engine/experiment/hhfm/hhfm_test.go

@@ -112,9 +112,6 @@ func TestSuccess(t *testing.T) {
 	if request.T != 0 {
 		t.Fatal("invalid Requests[0].T")
 	}
-	if request.TransactionID != 0 {
-		t.Fatal("invalid Requests[0].TransactionID")
-	}
 	if tk.SOCKSProxy != nil {
 		t.Fatal("invalid SOCKSProxy")
 	}
@@ -223,9 +220,6 @@ func TestCancelledContext(t *testing.T) {
 	if request.T != 0 {
 		t.Fatal("invalid Requests[0].T")
 	}
-	if request.TransactionID != 0 {
-		t.Fatal("invalid Requests[0].TransactionID")
-	}
 	if tk.SOCKSProxy != nil {
 		t.Fatal("invalid SOCKSProxy")
 	}