Minor LibraryMonitor improvements

* Enable nullable
* Add a fast return to ReportFileSystemChanged when path should be ignored
* Use Span overloads of Path.* functions where possible
* IFileSystem: remove NormalizePath as Path.TrimEndingDirectorySeparator already checks if it's a root path

MediaBrowser.Model/IO/IFileSystem.cs

@@ -116,13 +116,6 @@ namespace MediaBrowser.Model.IO
         /// <returns><c>true</c> if [contains sub path] [the specified parent path]; otherwise, <c>false</c>.</returns>
         bool ContainsSubPath(string parentPath, string path);
 
-        /// <summary>
-        /// Normalizes the path.
-        /// </summary>
-        /// <param name="path">The path.</param>
-        /// <returns>System.String.</returns>
-        string NormalizePath(string path);
-
         /// <summary>
         /// Gets the file name without extension.
         /// </summary>