I think you would need to use a combination of these 2 api calls:

1. Get deployment groups
2. Delete agent

$agents = Invoke-RestMethod -uri 'http://dev.azure.com/{organization}/_apis/distributedtask/pools/29/agents' -Method Get -UseDefaultCredentials
$agents.value |
    Where-Object { $_.status -eq 'offline' } |
    ForEach-Object {
        Invoke-RestMethod -uri "http://dev.azure.com/{organization}/_apis/distributedtask/pools/29/agents/$($_.id)?api-version=4.1" -Method Delete -UseDefaultCredentials
    }

Some assumptions for this solution:

1. You are looking for build agents
2. You know the id of the pool you are looking for already. You can get to this programatically also, or just loop through all pools if you want
3. You don't have any issues deleting any offline agents

Note: I'm using Azure DevOps Server, so replace the -UseDefaultCredentials with your authorization.