[CI] Remove caching of api-docs (#186050)

## Summary
When building the VM image, especially concerning the cache-warmup step
(https://github.com/elastic/ci-agent-images/pull/736) this step will
fail, because it's running on agents that won't have access to this
repository ([see this
build](https://buildkite.com/elastic/ci-vm-images/builds/5275#01900893-db68-4242-b73e-cda24df6e20c)).

We're probably not losing much by not having this repo cached, or if we
need it, we can always build the VM image with this repo cloned, and use
it through the git-mirrors.

.buildkite/scripts/packer_cache.sh

@@ -17,6 +17,3 @@ done
 for version in $(cat versions.json | jq -r '.versions[].version'); do
   node x-pack/plugins/security_solution/scripts/endpoint/agent_downloader --version "$version"
 done
-
-echo "--- Cloning repos for docs build"
-node scripts/validate_next_docs --clone-only