This should be removed. 'use-delay-panners' is too specific. There should (and will be) be generic code to select _some_ default panner as default.

A patch to add delay-based panners should only add files in `libs/panners/`  moving the delaylines to libardour might be OK to reduce code-duplication. but changing `panner_shell` and `panner_manager` is out.