Remove useless const qualifier

Claiming that the typevar argument to DefineCompositeType() is const
was a plain lie.  A similar case in DefineVirtualRelation() was
already changed in passing in commit 1575fbcb.  Also clean up the now
unnecessary casts that used to cast away the const.

src/backend/commands/typecmds.c

@@ -1980,7 +1980,7 @@ AssignTypeArrayOid(void)
  *-------------------------------------------------------------------
  */
 Oid
-DefineCompositeType(const RangeVar *typevar, List *coldeflist)
+DefineCompositeType(RangeVar *typevar, List *coldeflist)
 {
 	CreateStmt *createStmt = makeNode(CreateStmt);
 	Oid			old_type_oid;
@@ -1991,7 +1991,7 @@ DefineCompositeType(const RangeVar *typevar, List *coldeflist)
 	 * now set the parameters for keys/inheritance etc. All of these are
 	 * uninteresting for composite types...
 	 */
-	createStmt->relation = (RangeVar *) typevar;
+	createStmt->relation = typevar;
 	createStmt->tableElts = coldeflist;
 	createStmt->inhRelations = NIL;
 	createStmt->constraints = NIL;

src/backend/commands/view.c

@@ -254,7 +254,7 @@ DefineVirtualRelation(RangeVar *relation, List *tlist, bool replace,
 		 * now set the parameters for keys/inheritance etc. All of these are
 		 * uninteresting for views...
 		 */
-		createStmt->relation = (RangeVar *) relation;
+		createStmt->relation = relation;
 		createStmt->tableElts = attrList;
 		createStmt->inhRelations = NIL;
 		createStmt->constraints = NIL;

src/include/commands/typecmds.h

@@ -25,7 +25,7 @@ extern void DefineDomain(CreateDomainStmt *stmt);
 extern void DefineEnum(CreateEnumStmt *stmt);
 extern void DefineRange(CreateRangeStmt *stmt);
 extern void AlterEnum(AlterEnumStmt *stmt);
-extern Oid	DefineCompositeType(const RangeVar *typevar, List *coldeflist);
+extern Oid	DefineCompositeType(RangeVar *typevar, List *coldeflist);
 extern Oid	AssignTypeArrayOid(void);
 
 extern void AlterDomainDefault(List *names, Node *defaultRaw);