[arch][microblaze] remove microblaze and all traces of it

Alas it isn't really useful anymore since Xilinx has moved onto RISC-V
for all new products and there's nothing really that LK was ever used
for on this architecture except running on qemu (and once on an fpga for
a demo).

It's a pretty neat little architecture though.

Will set a tag in the repo in case it ever needs to be resurrected.