irqchip/loongson-htpic: Remove unneeded select of I8259

LOONGSON_HTPIC depends on MACH_LOONGSON64 and MACH_LOONGSON64 already
selects I8259 in arch/mips/Kconfig, so no need to select I8259 again
when config LOONGSON_HTPIC.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1594087972-21715-3-git-send-email-yangtiezhu@loongson.cn