Remove rm_safe_restartpoint machinery.

It is no longer used, none of the resource managers have multi-record
actions that would make it unsafe to perform a restartpoint.

Also don't allow rm_cleanup to write WAL records, it's also no longer
required. Move the call to rm_cleanup routines to make it more symmetric
with rm_startup.

src/include/access/xlog_internal.h

@@ -248,7 +248,6 @@ typedef struct RmgrData
 	void		(*rm_desc) (StringInfo buf, uint8 xl_info, char *rec);
 	void		(*rm_startup) (void);
 	void		(*rm_cleanup) (void);
-	bool		(*rm_safe_restartpoint) (void);
 } RmgrData;
 
 extern const RmgrData RmgrTable[];