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.