Remove too generically named MissingPtr typedef.

As there's only a single user of the typedef in the entire codebase,
just use the underlying struct directly.

Per complaint from Alvaro Herrera

Author: Andres Freund
Discussion: https://postgr.es/m/201901141836.oxtm4uzc63j3@alvherre.pgsql

src/include/access/tupdesc.h

@@ -25,8 +25,6 @@ typedef struct attrDefault
 	char	   *adbin;			/* nodeToString representation of expr */
 } AttrDefault;
 
-typedef struct attrMissing *MissingPtr;
-
 typedef struct constrCheck
 {
 	char	   *ccname;
@@ -40,7 +38,7 @@ typedef struct tupleConstr
 {
 	AttrDefault *defval;		/* array */
 	ConstrCheck *check;			/* array */
-	MissingPtr	missing;		/* missing attributes values, NULL if none */
+	struct attrMissing *missing;		/* missing attributes values, NULL if none */
 	uint16		num_defval;
 	uint16		num_check;
 	bool		has_not_null;