Remove the custom_variable_classes parameter.

This variable provides only marginal error-prevention capability (since
it can only check the prefix of a qualified GUC name), and the consensus
is that that isn't worth the amount of hassle that maintaining the setting
creates for DBAs.  So, let's just remove it.

With this commit, the system will silently accept a value for any qualified
GUC name at all, whether it has anything to do with any known extension or
not.  (Unqualified names still have to match known built-in settings,
though; and you will get a WARNING at extension load time if there's an
unrecognized setting with that extension's prefix.)

There's still some discussion ongoing about whether to tighten that up and
if so how; but if we do come up with a solution, it's not likely to look
anything like custom_variable_classes.

src/pl/plperl/expected/plperl_init.out

@@ -1,5 +1,5 @@
 -- test plperl.on_plperl_init errors are fatal
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperl_init
 LOAD 'plperl';
 SET SESSION plperl.on_plperl_init = ' system("/nonesuch") ';
 SHOW plperl.on_plperl_init;

src/pl/plperl/expected/plperl_shared.out

@@ -1,6 +1,6 @@
 -- test plperl.on_plperl_init via the shared hash
 -- (must be done before plperl is first used)
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperl_init
 LOAD 'plperl';
 -- testing on_plperl_init gets run, and that it can alter %_SHARED
 SET plperl.on_plperl_init = '$_SHARED{on_init} = 42';

src/pl/plperl/expected/plperlu.out

@@ -1,6 +1,6 @@
 -- Use ONLY plperlu tests here. For plperl/plerlu combined tests
 -- see plperl_plperlu.sql
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperlu_init
 LOAD 'plperl';
 -- Test plperl.on_plperlu_init gets run
 SET plperl.on_plperlu_init = '$_SHARED{init} = 42';

src/pl/plperl/sql/plperl_init.sql

@@ -1,6 +1,6 @@
 -- test plperl.on_plperl_init errors are fatal
 
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperl_init
 LOAD 'plperl';
 
 SET SESSION plperl.on_plperl_init = ' system("/nonesuch") ';

src/pl/plperl/sql/plperl_shared.sql

@@ -1,7 +1,7 @@
 -- test plperl.on_plperl_init via the shared hash
 -- (must be done before plperl is first used)
 
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperl_init
 LOAD 'plperl';
 
 -- testing on_plperl_init gets run, and that it can alter %_SHARED

src/pl/plperl/sql/plperlu.sql

@@ -1,7 +1,7 @@
 -- Use ONLY plperlu tests here. For plperl/plerlu combined tests
 -- see plperl_plperlu.sql
 
--- Avoid need for custom_variable_classes = 'plperl'
+-- Must load plperl before we can set on_plperlu_init
 LOAD 'plperl';
 
 -- Test plperl.on_plperlu_init gets run