[FIX] component: remove stale fiber in scheduler

Owl takes care of not rendering a component that will be unmounted
immediately after, and it works.  However, it also should clean
up the newly added fiber.

src/component/component.ts

@@ -340,6 +340,7 @@ export class Component<T extends Env, Props extends {}> {
         // we were mounted when render was called, but we aren't anymore, so we
         // were actually about to be unmounted ; we can thus forget about this
         // fiber
+        fiber.isCompleted = true;
         __owl__.currentFiber = null;
       }
     });

tests/component/component.test.ts

@@ -5052,6 +5052,8 @@ describe("unmounting and remounting", () => {
     await nextTick(); // wait for changes to be detected before remounting
     await widget.mount(fixture);
     expect(fixture.innerHTML).toBe("<div>3</div>");
+    // we want to make sure that there are no remaining tasks left at this point.
+    expect(Component.scheduler.tasks.length).toBe(0);
   });
 
   test("sub component is still active after being unmounted and remounted", async () => {