powerpc/mm: Remove unused variable declaration

Since commit 0034d395 ("powerpc/mm/hash64: Map all the kernel
regions in the same 0xc range") __kernel_virt_size is not used
anymore.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -283,7 +283,6 @@ static inline unsigned int ioremap_max_order(void)
 #define IOREMAP_MAX_ORDER ioremap_max_order()
 
 extern unsigned long __kernel_virt_start;
-extern unsigned long __kernel_virt_size;
 extern unsigned long __kernel_io_start;
 extern unsigned long __kernel_io_end;
 #define KERN_VIRT_START __kernel_virt_start