http: remove unused and broken 'content-len' logic

http: remove unused and broken 'content-len' logic

The HTTP tracking code would parse the content lenght and store it
in the TX user data. It didn't take the possibility or errors into
account though, leading to a possible negative int being cases to
unsigned int. Luckily, the result was unused.

This patch simply removes the offending code.

Reported-by: The Yahoo pentest team