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

This reverts commit 025d7f6.

Author: fluffynuts

src/log4net/Core/LoggingEvent.cs

@@ -922,8 +922,6 @@ public string ThreadName
         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 @@ private static string TryGetCurrentUserName()
 
             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)
             {