fix wrongly done validation of removal of desired feature properties

* it was enforced that "required" properties could not be removed
* this does not make sense for desired properties, as they are all optional by convention

Author: thjaeckle

things/service/src/main/java/org/eclipse/ditto/things/service/persistence/actors/strategies/commands/DeleteFeatureDesiredPropertiesStrategy.java

@@ -13,6 +13,7 @@
 package org.eclipse.ditto.things.service.persistence.actors.strategies.commands;
 
 import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionStage;
 
 import javax.annotation.Nullable;
@@ -68,19 +69,9 @@ protected CompletionStage<DeleteFeatureDesiredProperties> performWotValidation(
             @Nullable final Thing previousThing,
             @Nullable final Thing previewThing
     ) {
-        return wotThingModelValidator.validateFeatureScopedDeletion(
-                Optional.ofNullable(previousThing)
-                        .flatMap(Thing::getDefinition)
-                        .orElse(null),
-                Optional.ofNullable(previousThing)
-                        .flatMap(Thing::getFeatures)
-                        .flatMap(f -> f.getFeature(command.getFeatureId()))
-                        .flatMap(Feature::getDefinition)
-                        .orElse(null),
-                command.getFeatureId(),
-                command.getResourcePath(),
-                command.getDittoHeaders()
-        ).thenApply(aVoid -> command);
+        // it is always ok to delete a feature's desired properties - no need to validate for e.g. required properties,
+        //  as they do not apply for desired properties
+        return CompletableFuture.completedFuture(command);
     }
 
     private Optional<Feature> extractFeature(final DeleteFeatureDesiredProperties command,

things/service/src/main/java/org/eclipse/ditto/things/service/persistence/actors/strategies/commands/DeleteFeatureDesiredPropertyStrategy.java

@@ -13,6 +13,7 @@
 package org.eclipse.ditto.things.service.persistence.actors.strategies.commands;
 
 import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionStage;
 
 import javax.annotation.Nullable;
@@ -71,19 +72,9 @@ protected CompletionStage<DeleteFeatureDesiredProperty> performWotValidation(
             @Nullable final Thing previousThing,
             @Nullable final Thing previewThing
     ) {
-        return wotThingModelValidator.validateFeatureScopedDeletion(
-                Optional.ofNullable(previousThing)
-                        .flatMap(Thing::getDefinition)
-                        .orElse(null),
-                Optional.ofNullable(previousThing)
-                        .flatMap(Thing::getFeatures)
-                        .flatMap(f -> f.getFeature(command.getFeatureId()))
-                        .flatMap(Feature::getDefinition)
-                        .orElse(null),
-                command.getFeatureId(),
-                command.getResourcePath(),
-                command.getDittoHeaders()
-        ).thenApply(aVoid -> command);
+        // it is always ok to delete a feature's desired properties - no need to validate for e.g. required properties,
+        //  as they do not apply for desired properties
+        return CompletableFuture.completedFuture(command);
     }
 
     private Optional<Feature> extractFeature(final DeleteFeatureDesiredProperty command, final @Nullable Thing thing) {