powerpc/perf: Remove PM_BR_CMPL_ALT from power9 event list

PM_BR_CMPL_ALT event is not supported, remove it from the power9 event
list.

Fixes: 24bedcb7 ("powerpc/perf: Fix branch event code for power9")
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>