MIPS: Loongson: Remove out-of-date board-specific kgdb source code

Since the re-implementation of kgdb by commit
88547001

 the platform-specific version
has become superfluous, remove it.

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