Google has taken out another feature from Gmail Labs.
Signature Tweaks is now a core feature of Gmail available directly from settings.
Google spoke about this feature:
Today, Signature Tweaks officially graduates from Gmail Labs! Signature Tweaks lets you set your signature to appear above the quoted text in replies, removing the standard “”-”" that separates the signature from the message body. This feature can be turned on by going to the General tab in your Settings.