Remove a pointless MSGBUFSIZE definition.

The same operation is already done in <m68k/param.h>.
luna68k is not imported yet in 1997 but see also:
http://mail-index.netbsd.org/source-changes/1997/09/20/0021.html

sys/arch/luna68k/include/param.h

@@ -1,4 +1,4 @@
-/* $NetBSD: param.h,v 1.13 2012/02/10 17:35:48 para Exp $ */
+/* $NetBSD: param.h,v 1.14 2019/12/28 12:34:35 isaki Exp $ */
 
 /*
  * Copyright (c) 1988 University of Utah.
@@ -53,10 +53,6 @@
 
 #define NPTEPG		(NBPG/(sizeof (pt_entry_t)))
 
-#ifndef MSGBUFSIZE
-#define MSGBUFSIZE	NBPG		/* default message buffer size */
-#endif
-
 /*
  * Minimum and maximum sizes of the kernel malloc arena in PAGE_SIZE-sized
  * logical pages.