[Security Solutions] Removes POC transforms (#129673)

## Summary

Removes the `metrics_entities` plugin and POC. As a different direction will be taken and people can look back at the git history for it as they see fit if they need to refer to it. Once it's re-added it it will be through an RFC process and re-discussed.

Earlier PR's which added the POC:

https://github.com/elastic/kibana/pull/96446
https://github.com/elastic/kibana/pull/104559

### Checklist

- [x] [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

packages/kbn-cli-dev-mode/src/get_server_watch_paths.test.ts

@@ -76,7 +76,6 @@ it('produces the right watch and ignore list', () => {
       <absolute path>/x-pack/plugins/lists/server/scripts,
       <absolute path>/x-pack/plugins/security_solution/scripts,
       <absolute path>/x-pack/plugins/security_solution/server/lib/detection_engine/scripts,
-      <absolute path>/x-pack/plugins/metrics_entities/server/scripts,
       <absolute path>/x-pack/plugins/uptime/e2e,
     ]
   `);

packages/kbn-cli-dev-mode/src/get_server_watch_paths.ts

@@ -66,7 +66,6 @@ export function getServerWatchPaths({ pluginPaths, pluginScanDirs }: Options) {
     fromRoot('x-pack/plugins/lists/server/scripts'),
     fromRoot('x-pack/plugins/security_solution/scripts'),
     fromRoot('x-pack/plugins/security_solution/server/lib/detection_engine/scripts'),
-    fromRoot('x-pack/plugins/metrics_entities/server/scripts'),
     fromRoot('x-pack/plugins/uptime/e2e'),
   ];