They both have a completely different functions;
A filter pump will filter the water. A salt water system ensures that no chlorine has to be added to the water (only salt).
A saltwater system does not replace a filter pump, unless you have a saltwater system with a built-in filter pump. This system will then filter the water and regulate the quality of the water.
So if you have a salt water system without a filter pump, it is therefore necessary to connect a filter pump to it.