MIPS: Loongson: Remove existing early_printk implementation

Removes the existing implementation of early_printk for fulong.  The old
implementation was based on the outdated board-specific dbg_io.c.

Since commit 8d60a903 dbg_io.c is not longer
needed; it will be removed by the next patch, and a new implementation of
early_printk will be added later.

Signed-off-by: Wu Zhangjin <wuzj@lemote.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>