[Security Solution] Remove Optional label from Rule Schedule lookback…

… time field (#142375)

Relates to #141378

## Summary

Removes the "Optional" label from Rule Schedule lookback time field in the Rule Creation and Rule Editing pages.

## Screenshots

### Create Rule Page
**Before**
![image](https://user-images.githubusercontent.com/5354282/193306696-97b4ba91-4c93-406f-a69c-e0e3cb4553dd.png)

**After**
![image](https://user-images.githubusercontent.com/5354282/193306104-bffd2b4e-1131-4257-ac88-1f8cb3b1f5a5.png)



### Edit Rule Page
**Before**
![image](https://user-images.githubusercontent.com/5354282/193306747-7cc192a1-a33f-471d-8830-f85f4cce45e9.png)

**After**
![image](https://user-images.githubusercontent.com/5354282/193305897-8cfe029d-dc70-4d6d-9726-d00f5fde8e48.png)


### Checklist

Delete any items that are not applicable to this PR.

- [ ] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios
(https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive layout. (You can test this [in your browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)


Co-authored-by: Kibana Machine <[email protected]>
Co-authored-by: Dmitrii Shevchenko <[email protected]>

x-pack/plugins/security_solution/public/detections/components/rules/step_schedule_rule/schema.tsx

@@ -9,7 +9,6 @@
 
 import { i18n } from '@kbn/i18n';
 
-import { OptionalFieldLabel } from '../optional_field_label';
 import type { ScheduleStepRule } from '../../../pages/detection_engine/rules/types';
 import type { FormSchema } from '../../../../shared_imports';
 
@@ -35,7 +34,6 @@ export const schema: FormSchema<ScheduleStepRule> = {
         defaultMessage: 'Additional look-back time',
       }
     ),
-    labelAppend: OptionalFieldLabel,
     helpText: i18n.translate(
       'xpack.securitySolution.detectionEngine.createRule.stepScheduleRule.fieldAdditionalLookBackHelpText',
       {