Remove the unused local variable 'length' (#40693)

#30457 
In line 292, remove the unused variable 'length'.

Thanks.

src/libraries/System.Security.AccessControl/src/System/Security/AccessControl/SecurityDescriptor.cs

@@ -289,8 +289,6 @@ public void GetBinaryForm(byte[] binaryForm, int offset)
             // Populate the header
             //
 
-            int length = BinaryLength;
-
             byte rmControl =
                 ((this is RawSecurityDescriptor rsd) &&
                  ((ControlFlags & ControlFlags.RMControlValid) != 0)) ? (rsd.ResourceManagerControl) : (byte)0;