Unused code cleanup

[cniethammer]

Get rid of interfaces, methods, and functions that are not used anymore.

This should help improve code readability, compile times, and reduce binary size if compilers cannot optimize stuff away.

[HomesGH]

This issue is a good idea! There is a lot of mess in the code, especially regarding the thermostats, e.g. here and here. For the latter: Why is the warning only printed if unit tests are disabled? This FIXME for the thermostat is a bit worrying as this is in a central part of the code.

Also some output like this seems to be outdated.