riscv: Remove unnecessary declaration

max_low_pfn and min_low_pfn are declared in linux/memblock.h,
and it also is included in arch/riscv/mm/init.c, drop unnecessary
declaration.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/page.h

@@ -97,9 +97,6 @@ extern unsigned long pfn_base;
 #define ARCH_PFN_OFFSET		(PAGE_OFFSET >> PAGE_SHIFT)
 #endif /* CONFIG_MMU */
 
-extern unsigned long max_low_pfn;
-extern unsigned long min_low_pfn;
-
 #define __pa_to_va_nodebug(x)	((void *)((unsigned long) (x) + va_pa_offset))
 #define __va_to_pa_nodebug(x)	((unsigned long)(x) - va_pa_offset)