cleanup(ptx): remove redundant dialer definition (#644)

We can just use model.SimpleDialer here.

See https://github.com/ooni/probe/issues/1885.

internal/ptx/dependencies.go

@@ -1,17 +1,8 @@
 package ptx
 
 import (
-	"context"
-	"net"
-
 	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
-// UnderlyingDialer is the underlying dialer used for dialing.
-type UnderlyingDialer interface {
-	// DialContext behaves like net.Dialer.DialContext.
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
 // defaultLogger is the default silentLogger instance.
 var defaultLogger model.Logger = model.DiscardLogger