Accept DELETE in extension webhook route (#4408)

Author: Connor McMahon

src/WebJobs.Script.WebHost/Controllers/AdminController.cs

@@ -229,6 +229,7 @@ public async Task<HttpResponseMessage> SyncTriggers()
         [Route("admin/extensions/{name}/{*extra}")]
         [HttpGet]
         [HttpPost]
+        [HttpDelete]
         [AllowAnonymous]
         [RequiresRunningHost]
         public async Task<HttpResponseMessage> ExtensionWebHookHandler(string name, CancellationToken token)