Remove cmd/bash image

This image was previously responsible for invoking some specified bash
commands, typically cp and mkdir, but occasionally more exotic things
like setting up executable files for step scripts.

Ultimately this image existed merely to pass args to "sh -c" (not even
bash!), which can be done directly.

This change should be entirely transparent to end users.

Operators can override the image being used if they so desire, to point
to any image that contains `sh` (and `cp`, `mkdir`, `cat`, the usual
suspects).