Revert ":zap: cache resolved windows username when available"

This reverts commit 025d7f6ac197fc8ca92c776b14d9205d6b899168.
diff --git a/src/log4net/Core/LoggingEvent.cs b/src/log4net/Core/LoggingEvent.cs
index 8d89ff9..26b76ba 100644
--- a/src/log4net/Core/LoggingEvent.cs
+++ b/src/log4net/Core/LoggingEvent.cs
@@ -922,8 +922,6 @@
         public string UserName =>
             m_data.UserName ??= TryGetCurrentUserName() ?? SystemInfo.NotAvailableText;
 
-        private static string _cachedWindowsIdentityName;
-        
         private static string TryGetCurrentUserName()
         {
 #if (NETCF || SSCLI || NETSTANDARD1_3)
@@ -939,13 +937,10 @@
 
             try
             {
-                if (_cachedWindowsIdentityName is not null)
+                using (WindowsIdentity windowsIdentity = WindowsIdentity.GetCurrent())
                 {
-                    return _cachedWindowsIdentityName;
+                    return windowsIdentity?.Name ?? "";
                 }
-                
-                using var windowsIdentity = WindowsIdentity.GetCurrent();
-                return _cachedWindowsIdentityName = windowsIdentity?.Name ?? "";
             }
             catch (PlatformNotSupportedException)
             {