eliminate flux_child_watcher_create() from the public API

[garlick]

Problem: flux_child_watcher_create() is only used by internally by libsubprocess, and cannot be provided by libuv (#6492)

These are not super useful on their own, given that the Flux API provides the subprocess interface. Perhaps we could remove them from the public API for now, and then a port to libuv could provide an alternative implementation within libsubprocess.

See also: https://metacpan.org/dist/EV/view/libev/ev.pod#ev_child-watch-out-for-process-status-changes