Drop message severity

In the way we store the message registry, we store both Severity and
MessageSeverity.  Severity as a field is deprecated, and in every case
in every registry both fields have the same value.  We shouldn't
duplicate data in that way.  This commit changes the parse_registries.py
script to stop producing the Severity field into the struct.  The few
uses we have left are moved over to use MessageRegistry.

Tested:

Redfish service validator shows no errors on the
/redfish/v1/Registries tree.  Other errors present that were there
previously and are unchanged.

This saves a trivial amount: about 1kB on our compressed binary size.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ibbaf533dc59eb08365d6ed309aba16b54bc40ca1