Change UnitarySystem to not use heat recovery

After reading E+ IO reference more closely, the heat recovery appears
to only work with multi speed DX coil. So until this coil is
implemented in OpenStudio, I am commenting out the heat recovery code.
By commenting out the code, it will make it quick to enable the heat
recovery option once the coil is added to OS.

Since the heat recovery is no longer possible, I have disallowed adding
the unitary system to a plant loop.