Make temperature sensor type TT_NONE obsolete.

The requirement was simply neither obvious nor intuitive. Drawback
is, the indices of temperature sensors can now differ from these
of the heaters. That's easier to recognize for newbies, though.
Handle power_on() for heaters in heater.c.

This removes some instances in gcode_process.c and saves 26 bytes.
Move temp_all_zero() to heaters_all_zero().

We need the power supply for the heaters, after all, not for
the temperature sensors. Some heaters (e.g. fans) don't even have
a temp sensor.
gcode_process.c: remove the now obsolete M135.

The replacement is the now generalised M106.
gcode_process.c: remove M190 and M191.

These were pointless anyways, the power supply is handled fully
automatic in Teacup.
gcode_process.c: make the extruder optional and ...

... default to the first available device if the intended
device isn't available.
gcode_process.c: remove commented M113.
Fix auto-idle.

Powering on even with a heater set to zero doesn't work, as
temperature driven heaters are set to zero all the time. So,
the PSU was running all the time.

The bug was introduced with 569e3d504aafd1ffc8b279b5d7092970cef72287