Fix some comments related to library unloading

Library unloading has never been supported with its code removed in
ab02d702ef, and there were some comments still mentioning that it was
a possible operation.

ChangAo has noticed the incorrect references in dfmgr.c, while I have
noticed the other ones while scanning the rest of the tree for similar
mistakes.

Author: ChangAo Chen, Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/tencent_1D09840A1632D406A610C8C4E2491D74DB0A@qq.com

src/backend/utils/fmgr/dfmgr.c

@@ -125,7 +125,7 @@ load_external_function(const char *filename, const char *funcname,
 /*
  * This function loads a shlib file without looking up any particular
  * function in it.  If the same shlib has previously been loaded,
- * unload and reload it.
+ * we do not load it again.
  *
  * When 'restricted' is true, only libraries in the presumed-secure
  * directory $libdir/plugins may be referenced.
@@ -142,7 +142,7 @@ load_file(const char *filename, bool restricted)
 	/* Expand the possibly-abbreviated filename to an exact path name */
 	fullname = expand_dynamic_library_name(filename);
 
-	/* Load the shared library */
+	/* Load the shared library, unless we already did */
 	(void) internal_load_library(fullname);
 
 	pfree(fullname);