[SIEM] Removes an extra js-yaml entry from devDependencies

## Summary

Removes an extra entry for `js-yaml` in the `devDependencies` section of `x-pack/package.json`, which was introduced in #42804

The change in this PR did _not_ generate a new `yarn.lock`

### Checklist

- [ ] ~~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)~~

x-pack/package.json

@@ -141,7 +141,6 @@
     "jest-cli": "^24.8.0",
     "jest-styled-components": "^6.2.2",
     "jsdom": "^12.0.0",
-    "js-yaml": "^3.13.1",
     "madge": "3.4.4",
     "mocha": "3.5.3",
     "mocha-junit-reporter": "^1.23.1",