xen/arm: Remove the define INVALID_GFN from arch-arm/grant_table.h

The define INVALID_GFN is misplaced in arch-arm/grant_table.h and is
never used in neither in ARM nor in the common code.

Therefore we can safely remove it.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/include/asm-arm/grant_table.h

@@ -3,7 +3,6 @@
 
 #include <xen/grant_table.h>
 
-#define INVALID_GFN (-1UL)
 #define INITIAL_NR_GRANT_FRAMES 1
 
 void gnttab_clear_flag(unsigned long nr, uint16_t *addr);