arm64: remove the unnecessary arm64_swiotlb_init()

The commit 3690951f
(arm64: Use swiotlb late initialisation)
switches the DMA mapping code to swiotlb_tlb_late_init_with_default_size(),
the arm64_swiotlb_init() will not used anymore, so remove this function.
Signed-off-by: NDing Tianhong <dingtianhong@huawei.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/mm/mm.h

 extern void __init bootmem_init(void);
-extern void __init arm64_swiotlb_init(void);