kgdb,mips: remove unused kgdb_cpu_doing_single_step operations

The mips kgdb specific code does not support software or HW single
stepping so it should not implement

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
CC: linux-mips@linux-mips.org