[oe] squid: remove 2.x versions

[Frans Meulenbroeks]

Triggered by the recent commit from Roman:
http://cgit.openembedded.org/cgit.cgi/openembedded/commit/?id=b0f0b3ab87cf85c76498b57f8300539d133efa6f
There are various DoS issues resolved in 3 that are not resolved in
2.x (http://www.squid-cache.org/Advisories/)

2.6 is not a recommended version any more. 2.7 is lagging a version
(latest is stable9).
I propose to remove the 2.x versions and keep only 3.1.7

Frans