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;
...

```

For moving away from the endstops we do not have to disable them.
They are not tested anyway.

Use logic in Z_SAFE_HOMING

From
```
if (home_all_axis || homeZ) {
  if (home_all_axis) {
  ...
  home z
  }
  else if (homeZ) { // Don't need to Home Z twice
  home z
  }
}

```
to
```
if (home_all_axis || homeZ) {
  if (home_all_axis) {
  ...
  }
  home z
}
```

Always define MIN_Z_HEIGHT_FOR_HOMING even if 0
Always make a potential rise to `home_offset[Z_AXIS]` possible in G28.
Get rid of some very ugly constructs in MBL (ultralcd.cpp).

Differentiating made sense here when we could save a dozen lines of code,
but not for one line.

Since we do the actual homing now in the 'normal' routines `quick_home_xy()`
can be simplified to a relative simple diagonal move.