readings.html -- bad links

[Jan Hubicka]

> I already installed the patch below, but wonder whether one of you has
> updated URLs for the following?
> 
>   http://www-1.ibm.com/servers/s390/os390/bkserv/latest/z_arch.html
>   http://www.amd.com/K6/k6docs/pdf/21828a.pdf
> 
> (Would you mind either updating these links or removing them?)

Given the fact that AMD is constantly reordering their pages and that K6
is pretty old chip whose documentation is easy to find on AMD webpage, I
think we can just drop that link.

Honza