Remove leftover function prototype.

The prototype for inval_twophase_postcommit wasn't removed when it's definition
was removed in efc16ea5 / the initial HS commit.

Andres Freund

src/include/utils/inval.h

@@ -64,7 +64,4 @@ extern void CacheRegisterRelcacheCallback(RelcacheCallbackFunction func,
 
 extern void CallSyscacheCallbacks(int cacheid, uint32 hashvalue);
 
-extern void inval_twophase_postcommit(TransactionId xid, uint16 info,
-						  void *recdata, uint32 len);
-
 #endif   /* INVAL_H */