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 602a9ef5a7, 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/Ynsc9bRL1caUSBSE@paquier.xyz

src/backend/postmaster/pgarch.c

@@ -802,7 +802,7 @@ HandlePgArchInterrupts(void)
 			 * Ideally, we would simply unload the previous archive module and
 			 * load the new one, but there is presently no mechanism for
 			 * unloading a library (see the comment above
-			 * internal_unload_library()).  To deal with this, we simply restart
+			 * internal_load_library()).  To deal with this, we simply restart
 			 * the archiver.  The new archive module will be loaded when the new
 			 * archiver process starts up.
 			 */