games/gnuchess: remove duplicate plist entry

> Checking for items in pkg-plist which are not in STAGEDIR

Error: Missing: share/info/gnuchess.info

> Error: Plist issues found.

It's already added via INFO. Thanks to check-plist for providing
misleading error messages.

http://package21.nyi.freebsd.org/data/113amd64-default-qat/532906/logs/gnuchess-6.2.6.log