build: remove special definition of Runemax on Plan 9

The Bell Labs distribution now supports 21-bit runes.

R=golang-dev, akumar
CC=golang-dev
https://golang.org/cl/9249045

include/plan9/libc.h

@@ -2,16 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#define Runemax Plan9Runemax
 #include "/sys/include/libc.h"
-#undef Runemax
 #include "/sys/include/ctype.h"
 
-enum
-{
-	Runemax = 0x10FFFF, /* maximum rune value */
-};
-
 char*	getgoos(void);
 char*	getgoarch(void);
 char*	getgoroot(void);