Remove unused code

Author: nakamura-to

doma-processor/src/main/java/org/seasar/doma/internal/apt/annot/SingletonConfigAnnot.java

@@ -77,8 +77,6 @@
 
 public class DaoMetaFactory implements TypeElementMetaFactory<DaoMeta> {
 
-  private static final String SINGLETON_CONFIG_FIELD_NAME = "INSTANCE";
-
   private final Context ctx;
 
   private final List<BiFunction<TypeElement, ExecutableElement, QueryMetaFactory>> providers =

doma-processor/src/main/java/org/seasar/doma/internal/apt/meta/dao/DaoMetaFactory.java

@@ -123,7 +123,6 @@ public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContex
           invocationContext(SQLXMLFactoryDao.class),
           invocationContext(Issue82Dao.class),
           invocationContext(ResultStreamDao.class),
-          invocationContext(PlainSingletonConfigDao.class),
           invocationContext(SqlProcessorDao.class),
           invocationContext(OnlyDefaultMethodsExtendsDao.class),
           invocationContext(