refactor: remove express mapping strategy and tests

BREAKING CHANGE: `DefaultMappingStrategy` from `http-problem-details-mapper` or your own implementation of `IMappingStrategy` / `MappingStrategy` has to be used now.

Author: AlexZeitler

src/ExpressMappingStrategy.ts

@@ -1,7 +1,5 @@
-import { ExpressMappingStrategy } from './ExpressMappingStrategy'
 import { HttpProblemResponse } from './HttpProblemResponse'
 
 export {
-  ExpressMappingStrategy,
   HttpProblemResponse
 }