Remove non-functional code for unloading loadable modules.

Remove non-functional code for unloading loadable modules.

The code for unloading a library has been commented-out for over 12
years, ever since commit 602a9ef5a7c60151e10293ae3c4bb3fbb0132d03, and we're
no closer to supporting it now than we were back then.

Nathan Bossart, reviewed by Michael Paquier and by me.

Discussion: http://postgr.es/m/[email protected]