GIN readme is out of date

access/gin/README describes the Gin interface, but it hasn't been
updated since the change to extractQuery interface to allow "no query
can match" return value.

Since that interface documentation has been copied to the manual,
gin.sgml, section 52.2, which is up to date, how about we just remove it
from the README?

--  Heikki Linnakangas EnterpriseDB   http://www.enterprisedb.com