arm64: Remove unsued extern declaration init_mem_pgprot()

commit a501e32 ("arm64: Clean up the default pgprot setting")
left behind this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20230720143555.26044-1-yuehaibing@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: YueHaibing

arch/arm64/include/asm/mmu.h

@@ -64,7 +64,6 @@ extern void arm64_memblock_init(void);
 extern void paging_init(void);
 extern void bootmem_init(void);
 extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);
-extern void init_mem_pgprot(void);
 extern void create_mapping_noalloc(phys_addr_t phys, unsigned long virt,
 				   phys_addr_t size, pgprot_t prot);
 extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,