powerpc/pseries: Remove RTAS_POWERMGM_EVENTS

IBM bit 2 in the rtas event-scan and check-exception calls is
marked reserved in the PAPR, so remove it from our RAS code.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>