Fix minor thinko in ProcGlobalShmemSize().

There's no need to add space for startupBufferPinWaitBufId, because
it's part of the PROC_HDR object for which this function already
allocates space.

This has been wrong for a while, but the only consequence is that our
shared memory allocation is increased by 4 bytes, so no back-patch.

src/backend/storage/lmgr/proc.c

@@ -111,8 +111,6 @@ ProcGlobalShmemSize(void)
 	size = add_size(size, mul_size(MaxBackends, sizeof(PGPROC)));
 	/* ProcStructLock */
 	size = add_size(size, sizeof(slock_t));
-	/* startupBufferPinWaitBufId */
-	size = add_size(size, sizeof(NBuffers));
 
 	return size;
 }