The sick state of the director and the healthy callback are the same
thing coming from different directions: Either we query the status
dynamically or we already have it.

For layering directors, the health state is determined by their
backends, so having a director-layer sick state does not make much sense
and duplicates logic.

Also, the sick field duplicates admin_health to some extend (see
cache_director.c do_set_health).

This is also relevant in the context of streamlining the backend.list
output: admin_health "probe" only makes sense if backends actually do
have a probe (= some dynamically determined health state). It appears
streaight forward that the presense of a vdi_healthy_f callback is the
signal for dynamically determined health state.

So we move the sick field into VBE and retire VRT_SetHealth().

We also remove the ctx argument from VRT_SetChanged() because I
previously overlooked that it is required, for example, in probe code
where we got no ctx.

Ref #2896