tools: remove vexriscv_debug

tools: remove vexriscv_debug

This program is no longer needed.

The `openocd_vexriscv` package natively supports `etherbone`, and now
that the vexriscv debug module is available on Wishbone instead of as a
CSR, this module no longer works.

This change simplifies both tooling (because there is one fewer program
to run) and integration (because you don't need to modify your CSRs
anymore, just `register_mem()`.)

Signed-off-by: Sean Cross <sean@xobs.io>