refactor!: remove MasterDetailLayoutVariant (#9150)

## Summary
- Deletes the `MasterDetailLayoutVariant` enum and drops
`HasThemeVariant<MasterDetailLayoutVariant>` from `MasterDetailLayout`.
- The sole variant `AURA_INSET_DRAWER` is being removed from the
underlying web component in vaadin/web-components#11503, leaving the
Flow-side variant enum with no values, so both it and the mixin are no
longer needed.

## Test plan
- [x] `mvn test -pl
vaadin-master-detail-layout-flow-parent/vaadin-master-detail-layout-flow`
passes (38/38)
- [x] `grep -r "MasterDetailLayoutVariant"
vaadin-master-detail-layout-flow-parent/` → no matches

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: web-padawan

vaadin-master-detail-layout-flow-parent/vaadin-master-detail-layout-flow/src/main/java/com/vaadin/flow/component/masterdetaillayout/MasterDetailLayout.java

@@ -36,7 +36,6 @@
 import com.vaadin.flow.component.dependency.JsModule;
 import com.vaadin.flow.component.dependency.NpmPackage;
 import com.vaadin.flow.component.page.PendingJavaScriptResult;
-import com.vaadin.flow.component.shared.HasThemeVariant;
 import com.vaadin.flow.component.shared.SlotUtils;
 import com.vaadin.flow.dom.Element;
 import com.vaadin.flow.router.RouterLayout;
@@ -52,8 +51,8 @@
 @Tag("vaadin-master-detail-layout")
 @NpmPackage(value = "@vaadin/master-detail-layout", version = "25.2.0-alpha7")
 @JsModule("@vaadin/master-detail-layout/src/vaadin-master-detail-layout.js")
-public class MasterDetailLayout extends Component implements HasSize,
-        HasThemeVariant<MasterDetailLayoutVariant>, RouterLayout {
+public class MasterDetailLayout extends Component
+        implements HasSize, RouterLayout {
 
     public static final String MASTER_SLOT = "";
     public static final String DETAIL_PLACEHOLDER_SLOT = "detail-placeholder";

vaadin-master-detail-layout-flow-parent/vaadin-master-detail-layout-flow/src/main/java/com/vaadin/flow/component/masterdetaillayout/MasterDetailLayoutVariant.java

@@ -28,7 +28,6 @@
 import com.vaadin.flow.component.Text;
 import com.vaadin.flow.component.Unit;
 import com.vaadin.flow.component.html.Div;
-import com.vaadin.flow.component.shared.HasThemeVariant;
 import com.vaadin.tests.EnableFeatureFlagExtension;
 import com.vaadin.tests.MockUIExtension;
 
@@ -429,12 +428,6 @@ void addDetailEscapePressListener_verifyListenerTriggered() {
         Mockito.verify(listener).onComponentEvent(detailEscapePressEvent);
     }
 
-    @Test
-    void implementsHasThemeVariant() {
-        Assertions.assertTrue(HasThemeVariant.class
-                .isAssignableFrom(MasterDetailLayout.class));
-    }
-
     private void assertMasterContent(Component component) {
         Assertions.assertEquals("",
                 component.getElement().getAttribute("slot"));