MIPS: remove deprecated qemu_mips board

Remove qemu_mips boards because DM migration doesn't make sense.
The board support for qemu_mips is already marked as deprecated
in Qemu in favour of the Malta board. Also qemu_mips support
has been removed from Linux a long time ago.

The official replacement is the Malta board. The same Malta U-Boot
image can be used with Qemu and on physical hardware.
All combinations of Big Endian and Little Endian as well as 32 bit
and 64 bit are supported.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>