pseries: Remove redundant setting of mc->name for pseries-2.5 machine

pseries: Remove redundant setting of mc->name for pseries-2.5 machine

98cec76 "machine: Set MachineClass::name automatically" removed the setting
of mc->name for the pseries machine types, since it can be derived
automatically from the type names constructed with MACHINE_TYPE_NAME().

Unfortunately fb0fc8f "spapr: Create pseries-2.5 machine" went in later and
brought one of them back.

This removes it again.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>