nommu: Remove the memory_start/end variables from ARM page-nommu.h

These variables do not seem to be used anywhere in the kernel.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/include/asm/page-nommu.h

@@ -43,7 +43,4 @@ typedef unsigned long pgprot_t;
 #define __pmd(x)        (x)
 #define __pgprot(x)     (x)
 
-extern unsigned long memory_start;
-extern unsigned long memory_end;
-
 #endif