Move down to a safe height after G28 for DELTAs.

- Especially in `gcode_T`
- In `update_software_endstops`
- Some tweaks to observe

Don't deploy/stow the probe if z-min-endstop is used for homing.
Don't apply `zprobe_zoffset` if probe was not used.

Adjust wait_for_cooling slope
and drop mintemp for cooling.

See
https://github.com/MarlinFirmware/Marlin/pull/4169#issuecomment-231544532

when no known axis needs to be homed.

Most changes are only caused from altering the indentation.
```
if (axis == X_AXIS ? HOMEAXIS_DO(X) : axis == Y_AXIS ? HOMEAXIS_DO(Y) : axis == Z_AXIS ? HOMEAXIS_DO(Z) : 0) {
  ...
}

to

if (!(axis == X_AXIS ? HOMEAXIS_DO(X) : axis == Y_AXIS ? HOMEAXIS_DO(Y) : axis == Z_AXIS ? HOMEAXIS_DO(Z) : 0)) return;
...

```