MIPS: Loongson64: Remove not used pci.c

After commit 6423e59a ("MIPS: Loongson64: Switch to generic PCI
driver"), arch/mips/loongson64/pci.c is not used any more, remove it.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>