I know this is an old post, but it was one of the few that came up with google so I'm going to line out what I was able to do using the API. I'm on version 1.2.
I downloaded Postman to make the actual api requests. Then I used the Google Chrome Dev Tools -> Network and added an environment and deleted it to get the API details.
The reason for this is if you use the API to delete a request (I had one that wouldn't error out, even after rebooting vRSLCM) it leaves behind the environment but its not in the GUI at all.
So delete the request first via these instructions:
vRealize Automation Forum - VMware {code}
Then use:
GET: https://<IP OR FQDN of vRSLCM>/lcm/api/v1/view/environment
To get the ID of the hidden environment.
With header "content-type: application/json"
and header "x-xenon-auth-token: <your token>"
Then:
POST: https://<IP or FQDN of vRSLCM>lcm/api/delete/env/
To delete the environment. Put the ID in the BODY section in RAW format
With header "content-type: text/plain"
and header "x-xenon-auth-token: <your token>"
For all these to work you'll need the "x-xenon-auth-token" header
Which you can get here: https://<IP or FQDN of vRSLCM>/api/#/authorization/login
Click the very first "POST" (Authorize User) then in the new expanded box click "Try It Out" and then "Execute" and your x-xenon-auth-token will be the long string in the bottom.
Good luck!