phosphor-pid-control dropping libconfig support

[Patrick Venture]

In favor of supporting json, we're dropping libconfig support for
phosphor-pid-control.  One doesn't use libconfig for normal builds of
the daemon, there is a yaml configuration (or dbus configuration).  So
this should only impact a team using the libconfig option to test
different coefficients.

If you're currently using libconfig to test different coefficients,
let me know and I can help you transition to using json.

Patrick