Refactor ALTER some-obj RENAME implementation

Remove duplicate implementations of catalog munging and miscellaneous
privilege checks.  Instead rely on already existing data in
objectaddress.c to do the work.

Author: KaiGai Kohei, changes by me
Reviewed by: Robert Haas, Álvaro Herrera, Dimitri Fontaine

src/include/commands/conversioncmds.h

@@ -18,6 +18,5 @@
 #include "nodes/parsenodes.h"
 
 extern Oid CreateConversionCommand(CreateConversionStmt *parsetree);
-extern Oid RenameConversion(List *name, const char *newname);
 
 #endif   /* CONVERSIONCMDS_H */