Do not call UseContentRoot() on the web host builder

This call has nothing to do with static web content and should not have ever been called with the web content path: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/?view=aspnetcore-3.1&tabs=windows#content-root
Since this call is removed, the ContentRoot property is also removed from ApplicationHost as it is no longer needed.
Finally, move validation of the static content path to the DashboardService where it is actually used.

Emby.Server.Implementations/ApplicationHost.cs

@@ -236,11 +236,6 @@ namespace Emby.Server.Implementations
         /// </summary>
         public int HttpsPort { get; private set; }
 
-        /// <summary>
-        /// Gets the content root for the webhost.
-        /// </summary>
-        public string ContentRoot { get; private set; }
-
         /// <summary>
         /// Gets the server configuration manager.
         /// </summary>
@@ -614,12 +609,6 @@ namespace Emby.Server.Implementations
             DiscoverTypes();
 
             await RegisterServices(serviceCollection, startupConfig).ConfigureAwait(false);
-
-            ContentRoot = ServerConfigurationManager.Configuration.DashboardSourcePath;
-            if (string.IsNullOrEmpty(ContentRoot))
-            {
-                ContentRoot = ServerConfigurationManager.ApplicationPaths.WebPath;
-            }
         }
 
         public async Task ExecuteWebsocketHandlerAsync(HttpContext context, Func<Task> next)