Remove contrib version of rtree_gist --- now in core system.

contrib/Makefile

contrib/README

contrib/rtree_gist/Makefile

contrib/rtree_gist/README.rtree_gist

contrib/rtree_gist/bench/bench.pl

contrib/rtree_gist/bench/create_test.pl

contrib/rtree_gist/data/test_box.data

contrib/rtree_gist/expected/rtree_gist.out

contrib/rtree_gist/rtree_gist.c

contrib/rtree_gist/rtree_gist.sql.in

contrib/rtree_gist/sql/rtree_gist.sql