docs: update modules/README.

* modules/README: Update to reflect current state.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
modules: remove ancient broken perl module.

* modules/perl.c, modules/perl.m4: Remove.
* po/POTFILES.in: Remove reference to perl.c.
* Makefile.am: Remove commented out rules for building perl.so.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>