Remove reference to mirrors in index.html files

There are places where references to mirror sites should be kept in like
in the FAQ (currently FAQ 6.2 with the title 'May I mirror the IOCCC
website?' and with the named anchor 'mirrors', 'website_mirrors' and
'website_mirroring').

2013/README.md

@@ -12,10 +12,6 @@ Some ANSI C compilers are not quite as good as they should be.  If
 yours is lacking, you may need to compile using gcc instead of your
 local compiler.
 
-The IOCCC has a website and now has a number of international mirrors.
-The primary website can be found at [www.ioccc.org](https://www.ioccc.org).
-
-
 
 ## Remarks on some of the entries