There is a way to turn off the steering assistance, it's just a bit obscure. The assistance itself is there for keyboard users (though hardly anyone plays these games with a keyboard, for a good reason).
I think you should just write to the game developers to make them develop an option to turn the feature on or off.
It does not bother me anymore. It used to at some point but not anymore from what I have seen so far. I guess for each one of us it could be definitely different.
I don't think there is a fix or mod at this time to disable the steering assistance in F1 2013. The developers included this feature to appeal to a wider audience, and unfortunately, it takes away from the overall experience of the game. Kinda bummer!