Node.js SDK 3.0.0 docs: remove reference to ping as it is currently not implemented

Remove mention of ping in this doc https://docs.couchbase.com/nodejs-sdk/3.0/concept-docs/health-check.html

This is because ping is currently not implemented in the C SDK yet, and thus Node.js SDK won't have it. 

In future release the ping will be included, which then we can include the ping. It doesn't seem likely it will be included in 3.0.1 either. 

Also, the how-to's docs are empty https://docs.couchbase.com/nodejs-sdk/3.0/howtos/health-check.html, not sure what we plan on adding there or if we just need to delete.