Eventually remove `tests/xilinx/cocotb` directory

[nathanielnrn]

As mentioned in #1997, we should eventually remove the cocotb Makefile and python runners found here in favor of using the tests/yxi Makefile and harnesses.

Before this can happen, we need to get the AXI wrapper to be able to correctly interface with XRT, which is still only possible with the old verilog wrapper.