Redundant nullcheck of rs which is known to be null in AddConceptMapTypesChangeset

Because the rs variable is not actually used, just remove it.

In other words, remove this line https://github.com/openmrs/openmrs-core/blob/master/api/src/main/java/org/openmrs/util/databasechange/AddConceptMapTypesChangeset.java#L76

together with these https://github.com/openmrs/openmrs-core/blob/master/api/src/main/java/org/openmrs/util/databasechange/AddConceptMapTypesChangeset.java#L184-L191