Address review feedback on UnassignedBannerService (#8680)

* Introduce `UnassignedItemsBannerApiService`

* Delete `WebUnassignedItemsBannerService`

libs/angular/src/services/unassigned-items-banner.api.service.ts

@@ -0,0 +1,19 @@
+import { Injectable } from "@angular/core";
+
+import { ApiService } from "@bitwarden/common/abstractions/api.service";
+
+@Injectable({ providedIn: "root" })
+export class UnassignedItemsBannerApiService {
+  constructor(private apiService: ApiService) {}
+
+  async getShowUnassignedCiphersBanner(): Promise<boolean> {
+    const r = await this.apiService.send(
+      "GET",
+      "/ciphers/has-unassigned-ciphers",
+      null,
+      true,
+      true,
+    );
+    return r;
+  }
+}