obsolete indexing method "rtree"

We removed the indexing method "rtree" in version 8.3 and replaced it
with "gist". Until today there are some "rtree" heritages:

* "CREATE INDEX ... USING rtree ..." is valid syntax and leads to the
message "NOTICE:  substituting access method gist for obsolete
method rtree"
* The documentation refers to the method in ref/comments.sgml and
ref/create_index.sgml.
* The documentation uses the term "R-Tree" in gist.sgml, seg.sgml, and
xindex.sgml. That's ok because in this cases it refers to the
abstract concept of R-trees and not to the eliminated implementation.

Isn't it time to disallow this syntax and remove all references and
messages? The situation concerning index types, access methods, operator
classes, and index related extensions is complicate enough and hard to
explain to newcomers. Every simplification should be welcomed.

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Jürgen Purtz