[SIEM][Detection Engine] Removes deprecated keys from configuration

## Summary

Removes deprecated keys from configuration since the backend gives out where the index is located and it is based on spaces.

### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

~~- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)~~

~~- [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)~~

~~- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~~

~~- [ ] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios~~

~~- [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~~

### For maintainers

~~- [ ] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

~~- [ ] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

Author: FrankHassanabad

x-pack/legacy/plugins/siem/common/constants.ts

@@ -16,11 +16,6 @@ export const DEFAULT_TIME_RANGE = 'timepicker:timeDefaults';
 export const DEFAULT_REFRESH_RATE_INTERVAL = 'timepicker:refreshIntervalDefaults';
 export const DEFAULT_SIEM_TIME_RANGE = 'siem:timeDefaults';
 export const DEFAULT_SIEM_REFRESH_INTERVAL = 'siem:refreshIntervalDefaults';
-
-// DEPRECATED: THIS WILL BE REMOVED VERY SOON AND IS NO LONGER USED ON THE BACKEND
-// TODO: Remove this as soon as no code is left that is pulling data from it.
-export const DEFAULT_SIGNALS_INDEX_KEY = 'siem:defaultSignalsIndex';
-
 export const DEFAULT_SIGNALS_INDEX = '.siem-signals';
 export const DEFAULT_MAX_SIGNALS = 100;
 export const DEFAULT_SEARCH_AFTER_PAGE_SIZE = 100;

x-pack/legacy/plugins/siem/index.ts

@@ -26,7 +26,6 @@ import {
   DEFAULT_TO,
   DEFAULT_SIGNALS_INDEX,
   SIGNALS_INDEX_KEY,
-  DEFAULT_SIGNALS_INDEX_KEY,
 } from './common/constants';
 import { defaultIndexPattern } from './default_index_pattern';
 import { initServerWithKibana } from './server/kibana.index';
@@ -106,20 +105,6 @@ export const siem = (kibana: any) => {
           category: ['siem'],
           requiresPageReload: true,
         },
-        // DEPRECATED: This should be removed once the front end is no longer using any parts of it.
-        // TODO: Remove this as soon as no code is left that is pulling data from it.
-        [DEFAULT_SIGNALS_INDEX_KEY]: {
-          name: i18n.translate('xpack.siem.uiSettings.defaultSignalsIndexLabel', {
-            defaultMessage: 'Elasticsearch signals index',
-          }),
-          value: DEFAULT_SIGNALS_INDEX,
-          description: i18n.translate('xpack.siem.uiSettings.defaultSignalsIndexDescription', {
-            defaultMessage:
-              '<p>Elasticsearch signals index from which outputted signals will appear by default</p>',
-          }),
-          category: ['siem'],
-          requiresPageReload: true,
-        },
         [DEFAULT_ANOMALY_SCORE]: {
           name: i18n.translate('xpack.siem.uiSettings.defaultAnomalyScoreLabel', {
             defaultMessage: 'Anomaly threshold',