Answer: no reason to dry space probes.
Many space probes are treated with dry heat. This is done for bacterial decontamination, not to dry moisture. See https://en.wikipedia.org/wiki/Planetary_protection for heat sterilization protocols. Viking Landers were treated with dry heat at 112 °C for 30 hours.
Off-gassing water in space is only a concern if the off-gassing vapor is in a sealed compartment capable of generating pressure. But if it is in a sealed container, it is protected from the vacuum. Kind of a reverse Catch 22.
Liquid water evaporates until the vapor pressure reaches equilibrium pressure characteristic for that temperature. The vapor pressure depends only on temperature.
Consider 2 bottles. One is filled with room air at 50% relative humidity (RH) , the other with water (and a bit of air). The air in the second bottle had 50% RH at the time it was filled, then water evaporated into the air space until it reaches the partial pressure of water vapor appropriate for the ambient temperature. The pressure gradient across the wall of the bottle is 0.000 bar in the first case and 0.005 bar in the second. In space, first bottle will have a pressure of 1.000 bar and the second 1.005bar (at room temperature).
So the presence of liquid water in a sealed compartment is insignificant. In vented spaces, the water vaporizes and disperses.