remove unused imports from generated code

[hduelme]

This PR removes unused imports and simplifies conditional imports in the template.
Changes

• Removed the following unused imports:
  • java.util.ArrayList;
  • javax.lang.model.element.ElementKind
  • javax.lang.model.element.VariableElement
  • javax.lang.model.util.AbstractAnnotationValueVisitor8
• he following imports are only required when gemValueInfos contains values. Instead of always including them, they are now conditionally added via an if statement in the FTL template:
  • java.util.HashMap
  • java.util.List
  • java.util.Map
  • javax.lang.model.element.AnnotationValue
  • javax.lang.model.element.ExecutableElement
  • javax.lang.model.type.TypeMirror
  • javax.lang.model.util.ElementFilter;
  • org.mapstruct.tools.gem.GemValue
• javax.lang.model.type.TypeMirror is already imported elsewhere and has therefore been excluded from the manual import list.