Update database.ts

Author: schmidt-sebastian

packages/firestore/src/api/database.ts

@@ -42,7 +42,6 @@ import { Document, MaybeDocument, NoDocument } from '../model/document';
 import { DocumentKey } from '../model/document_key';
 import { DeleteMutation, Mutation, Precondition } from '../model/mutation';
 import { FieldPath, ResourcePath } from '../model/path';
-import { JsonProtoSerializer } from '../remote/serializer';
 import { isServerTimestamp } from '../model/server_timestamps';
 import { refValue } from '../model/values';
 import { PlatformSupport } from '../platform/platform';