Redfish.md: Session no longer has Oem field

Remove the field that is no longer exposed by bmcweb.

Tested: no testing, documentation-only change.

Fixes: d678d4fc59f131cf9820c1783f86a4a6ede32995
Change-Id: I6ed07d35bfc0a4a205828773f854e56582058f46
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
diff --git a/Redfish.md b/Redfish.md
index cfee9d5..2ec6381 100644
--- a/Redfish.md
+++ b/Redfish.md
@@ -728,7 +728,6 @@
 
 - ClientOriginIPAddress
 - Description
-- Oem
 - UserName
 
 ### /redfish/v1/Systems/