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

src/backend/parser/gram.y

@@ -4460,7 +4460,7 @@ DropTableSpaceStmt: DROP TABLESPACE name
  *
  *		QUERY:
  *             CREATE EXTENSION extension
- *             [ WITH ] [ SCHEMA schema ] [ VERSION version ] [ FROM oldversion ]
+ *             [ WITH ] [ SCHEMA schema ] [ VERSION version ]
  *
  *****************************************************************************/
 
@@ -4500,7 +4500,10 @@ create_extension_opt_item:
 				}
 			| FROM NonReservedWord_or_Sconst
 				{
-					$$ = makeDefElem("old_version", (Node *)makeString($2), @1);
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("CREATE EXTENSION ... FROM is no longer supported"),
+							 parser_errposition(@1)));
 				}
 			| CASCADE
 				{