Remove support for upgrading extensions from "unpackaged" state.

Andres Freund pointed out that allowing non-superusers to run
"CREATE EXTENSION ... FROM unpackaged" has security risks, since
the unpackaged-to-1.0 scripts don't try to verify that the existing
objects they're modifying are what they expect.  Just attaching such
objects to an extension doesn't seem too dangerous, but some of them
do more than that.

We could have resolved this, perhaps, by still requiring superuser
privilege to use the FROM option.  However, it's fair to ask just what
we're accomplishing by continuing to lug the unpackaged-to-1.0 scripts
forward.  None of them have received any real testing since 9.1 days,
so they may not even work anymore (even assuming that one could still
load the previous "loose" object definitions into a v13 database).
And an installation that's trying to go from pre-9.1 to v13 or later
in one jump is going to have worse compatibility problems than whether
there's a trivial way to convert their contrib modules into extension
style.

Hence, let's just drop both those scripts and the core-code support
for "CREATE EXTENSION ... FROM".

Discussion: https://postgr.es/m/20200213233015.r6rnubcvl4egdh5r@alap3.anarazel.de

contrib/spi/Makefile

@@ -4,10 +4,10 @@ MODULES = autoinc insert_username moddatetime refint
 
 EXTENSION = autoinc insert_username moddatetime refint
 
-DATA = autoinc--1.0.sql autoinc--unpackaged--1.0.sql \
-       insert_username--1.0.sql insert_username--unpackaged--1.0.sql \
-       moddatetime--1.0.sql moddatetime--unpackaged--1.0.sql \
-       refint--1.0.sql refint--unpackaged--1.0.sql
+DATA = autoinc--1.0.sql \
+       insert_username--1.0.sql \
+       moddatetime--1.0.sql \
+       refint--1.0.sql
 PGFILEDESC = "spi - examples of using SPI and triggers"
 
 DOCS = $(addsuffix .example, $(MODULES))

contrib/spi/autoinc--unpackaged--1.0.sql

@@ -1,6 +0,0 @@
-/* contrib/spi/autoinc--unpackaged--1.0.sql */
-
--- complain if script is sourced in psql, rather than via CREATE EXTENSION
-\echo Use "CREATE EXTENSION autoinc FROM unpackaged" to load this file. \quit
-
-ALTER EXTENSION autoinc ADD function autoinc();

contrib/spi/insert_username--unpackaged--1.0.sql

@@ -1,6 +0,0 @@
-/* contrib/spi/insert_username--unpackaged--1.0.sql */
-
--- complain if script is sourced in psql, rather than via CREATE EXTENSION
-\echo Use "CREATE EXTENSION insert_username FROM unpackaged" to load this file. \quit
-
-ALTER EXTENSION insert_username ADD function insert_username();

contrib/spi/moddatetime--unpackaged--1.0.sql

@@ -1,6 +0,0 @@
-/* contrib/spi/moddatetime--unpackaged--1.0.sql */
-
--- complain if script is sourced in psql, rather than via CREATE EXTENSION
-\echo Use "CREATE EXTENSION moddatetime FROM unpackaged" to load this file. \quit
-
-ALTER EXTENSION moddatetime ADD function moddatetime();

contrib/spi/refint--unpackaged--1.0.sql

@@ -1,7 +0,0 @@
-/* contrib/spi/refint--unpackaged--1.0.sql */
-
--- complain if script is sourced in psql, rather than via CREATE EXTENSION
-\echo Use "CREATE EXTENSION refint FROM unpackaged" to load this file. \quit
-
-ALTER EXTENSION refint ADD function check_primary_key();
-ALTER EXTENSION refint ADD function check_foreign_key();