Remove the refint extension.

Remove the refint extension.

refint was sample code from the pre-built-in-FK era and has long
been documented as superseded by the built-in foreign key
mechanism.  Recent fixes (see commits b0b6196386, 8cfbdf8f4d,
260e97733b, 611756948e, 1fbe2066dc, and 1541d91d1c) made it clear
that the code has more issues than its sample-code value justifies.

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: solai v <solai.cdac@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAJTYsWUHq8Ohc6-N-xamOPYz-q3qUYMtwQX-1%3DZi%3D5N1Q_GSEQ%40mail.gmail.com