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.

MediaBrowser.Common/Configuration/IApplicationPaths.cs

@@ -1,3 +1,5 @@
+using MediaBrowser.Model.Configuration;
+
 namespace MediaBrowser.Common.Configuration
 {
     /// <summary>
@@ -12,9 +14,12 @@ namespace MediaBrowser.Common.Configuration
         string ProgramDataPath { get; }
 
         /// <summary>
-        /// Gets the path to the web UI resources folder
+        /// Gets the path to the web UI resources folder.
         /// </summary>
-        /// <value>The web UI resources path.</value>
+        /// <remarks>
+        /// This value is not relevant if the server is configured to not host any static web content. Additionally,
+        /// the value for <see cref="ServerConfiguration.DashboardSourcePath"/> takes precedence over this one.
+        /// </remarks>
         string WebPath { get; }
 
         /// <summary>