Remove leftover function declaration

Commit 9d9b9d46f3 removed the function (or rather, moved it to a
different source file and renamed it to SendCancelRequest), but forgot
the declaration in the header file.

src/include/postmaster/postmaster.h

@@ -63,8 +63,6 @@ extern int	MaxLivePostmasterChildren(void);
 
 extern bool PostmasterMarkPIDForWorkerNotify(int);
 
-extern void processCancelRequest(int backendPID, int32 cancelAuthCode);
-
 #ifdef WIN32
 extern void pgwin32_register_deadchild_callback(HANDLE procHandle, DWORD procId);
 #endif