The proprietary build can install them as normal C language functions,
with CREATE FUNCTION, instead.

In the passing, remove some unused QuickLZ debugging GUCs.

This doesn't yet get rid of all references to QuickLZ, unfortunately. The
GUC and reloption validation code still needs to know about it, so that
they can validate the options read from postgresql.conf, when starting up
postmaster. For the same reason, you cannot yet add custom compression
algorithms, besides quicklz, as an extension. But this is another step in
the right direction, anyway.
Co-authored-by: NJimmy Yih <jyih@pivotal.io>
Co-authored-by: NJoao Pereira <jdealmeidapereira@pivotal.io>