Preset: Remove “high”, “medium” and “low” values for key “voltage”

[sommerluk]

Various presets for the mapping of electrical power facilities support the key “voltage”. These presets propose not only some typical integer values, but also the values “high”, “medium” and “low”. The wiki page does not describe these values. These values are also almost not used in the database: voltage=high: 111, voltage=medium: 1024, voltage=low: 498. Much, much more integer values for key “voltage” in the database. Total usage: voltage=*: 949 723. I propose to remove “high”, “medium” and “low” from the presets.