I propose killing PL/Tcl's "modules" infrastructure

Over in
https://www.postgresql.org/message-id/alpine.DEB.2.11.1702251701030.3920@Sandal.Woodpecker
it's pointed out that pltcl_loadmod was never updated for the switch
to standard_conforming_strings (and the patch proposed there doesn't
begin to cover all the places that would need fixed for that).

This means that the "modules" functionality is entirely broken in any
installation that's got standard_conforming_strings turned on, which has
been the default since 9.1 and was possible long before that.

The fact that nobody has noticed seems to me to be clear proof that no one
is using this feature in the field.

Now, we could try to fix this bug, and add the regression test coverage
that the code clearly lacks, and upgrade the documentation about it from
its currently very sad state. But I think the right answer is just to
remove the feature altogether. It's evidently not being used, and it's
kind of insecure by design, and it would not be that hard for someone
to provide equivalent functionality entirely in userland if they really
wanted it.

Comments?

regards, tom lane