[Security Solution] Cleanup legacy serverless navigation (elastic#175214

)

## Summary

elastic#174944

Remove the legacy navigation components, chrome navigation tree, and
experimental feature.
This PR only removes unused code, It does not introduce any behavioral
change. The project navigation is already enabled.

---------

Co-authored-by: kibanamachine <[email protected]>

x-pack/plugins/security_solution_serverless/common/experimental_features.ts

@@ -17,9 +17,12 @@ export type ServerlessExperimentalFeatures = Record<
  */
 export const allowedExperimentalValues = Object.freeze({
   /**
-   * Enables the use of the of the product navigation from shared-ux package in the Security Solution app
+   * <Add a description of the feature here>
+   *
+   * [This is a fake feature key to showcase how to add a new serverless-specific experimental flag.
+   * It also prevents `allowedExperimentalValues` from being empty. It should be removed once a real feature is added.]
    */
-  platformNavEnabled: true,
+  _serverlessFeatureEnabled: false,
 });
 
 type ServerlessExperimentalConfigKeys = Array<keyof ServerlessExperimentalFeatures>;

x-pack/plugins/security_solution_serverless/public/navigation/index.ts

@@ -10,8 +10,7 @@ import type { CoreSetup } from '@kbn/core/public';
 import type { SecuritySolutionServerlessPluginSetupDeps } from '../types';
 import type { Services } from '../common/services';
 import { subscribeBreadcrumbs } from './breadcrumbs';
-import { getSecuritySideNavComponent } from './side_navigation';
-import { initProjectNavigation } from './project_navigation';
+import { initSideNavigation } from './side_navigation';
 import { projectAppLinksSwitcher } from './links/app_links';
 import { formatProjectDeepLinks } from './links/deep_links';
 import { enableManagementCardsLanding } from './management_cards';
@@ -27,21 +26,9 @@ export const setupNavigation = (
 export const startNavigation = (services: Services) => {
   const { serverless, management } = services;
   serverless.setProjectHome(APP_PATH);
+  initSideNavigation(services);
 
   enableManagementCardsLanding(services);
-
-  if (services.experimentalFeatures.platformNavEnabled) {
-    initProjectNavigation(services);
-  } else {
-    // TODO: Remove this else block as platform is enabled by default
-    // Issue: https://github.com/elastic/kibana/issues/174944
-
-    // const projectNavigationTree = new ProjectNavigationTree(services);
-    // projectNavigationTree.getChromeNavigationTree$().subscribe((chromeNavigationTree) => {
-    //   serverless.setNavigation({ navigationTree: chromeNavigationTree });
-    // });
-    serverless.setSideNavComponentDeprecated(getSecuritySideNavComponent(services));
-  }
   management.setIsSidebarEnabled(false);
 
   subscribeBreadcrumbs(services);