The following example deletes the subsite from the parent website based on the given url and returns true value. If the subsite is not available or problem in deleting the child site, this method returns false.

PnP.DeleteWeb.cs

//Author: Shantha Kumar T
//Assembly Reference Used: OfficeDevPnP.Core, Version=2.4.1605.0, Culture=neutral, PublicKeyToken=3751622786b357c2
//Supports: SharePoint Online, SharePoint 2013+

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;
using OfficeDevPnP.Core;

// Assembly Reference Used: OfficeDevPnP.Core, Version=2.4.1605.0, Culture=neutral, PublicKeyToken=3751622786b357c2
namespace SampleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            string siteUrl = "https://sharepointonline.sharepoint.com";           

            AuthenticationManager authManager = new AuthenticationManager();
            //Interactive Login to SharePoint site - Opens a Online signin page to authenticate the user
            var context = authManager.GetWebLoginClientContext(siteUrl);
	     //Deletes the child website with the specified subsite URL, from a parent Web, if it exists. Else the method return false value.
            bool success = context.Web.DeleteWeb("subsiteurl");

            if (success)
                Console.WriteLine("Site deleted successfully.");
            else
                Console.WriteLine("Problem in deleting the site.");

            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
            
        } 
    }
}