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

contrib/auto_explain/auto_explain.c

@@ -70,7 +70,7 @@ static bool current_query_sampled = false;
 	 (nesting_level == 0 || auto_explain_log_nested_statements) && \
 	 current_query_sampled)
 
-/* Saved hook values in case of unload */
+/* Saved hook values */
 static ExecutorStart_hook_type prev_ExecutorStart = NULL;
 static ExecutorRun_hook_type prev_ExecutorRun = NULL;
 static ExecutorFinish_hook_type prev_ExecutorFinish = NULL;

contrib/passwordcheck/passwordcheck.c

@@ -26,7 +26,7 @@
 
 PG_MODULE_MAGIC;
 
-/* Saved hook value in case of unload */
+/* Saved hook value */
 static check_password_hook_type prev_check_password_hook = NULL;
 
 /* passwords shorter than this will be rejected */

contrib/pg_stat_statements/pg_stat_statements.c

@@ -256,7 +256,7 @@ typedef struct pgssSharedState
 /* Current nesting depth of planner/ExecutorRun/ProcessUtility calls */
 static int	nesting_level = 0;
 
-/* Saved hook values in case of unload */
+/* Saved hook values */
 static shmem_request_hook_type prev_shmem_request_hook = NULL;
 static shmem_startup_hook_type prev_shmem_startup_hook = NULL;
 static post_parse_analyze_hook_type prev_post_parse_analyze_hook = NULL;

contrib/sepgsql/hooks.c

@@ -394,7 +394,7 @@ sepgsql_utility_command(PlannedStmt *pstmt,
 }
 
 /*
- * Module load/unload callback
+ * Module load callback
  */
 void
 _PG_init(void)