OpenRISC: Remove memory_start/end prototypes

OpenRISC does not have global memory_start and memory_end
symbols.
The prototypes are in vain.
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/include/asm/page.h

@@ -71,9 +71,6 @@ typedef struct page *pgtable_t;
 #define __pgd(x)	((pgd_t) { (x) })
 #define __pgprot(x)	((pgprot_t) { (x) })
 
-extern unsigned long memory_start;
-extern unsigned long memory_end;
-
 #endif /* !__ASSEMBLY__ */