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').

2006/README.md

@@ -15,9 +15,6 @@ local compiler.
 This year we included most of the information included by the submitters
 in the `README.md` files (that were used to build the `index.html` web pages).
 
-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