[fluidstate] Remove outdated interface setRelativeHumidity

* the function is unused in dumux core
* it was not test and therefore is outdated and doesn't fit the index
conventions in the fluid system anymore
* it is also not appropriate as an interface of the general fluid state
implementation and has a lot of very specific constraints such that is
it only useful in a specialized setting.

From now on: simply use the other setters to achieve the same effect.