powerpc: Remove redundant xics badness warning

While testing cpu offlining, we are regularly seeing the WARN_ON go off
in xics_ipi_dispatch. It can occur when an IPI gets sent to the CPU while
it is going offline. There is already a similar WARN_ON in the handlers
for PPC_MSG_CALL_FUNCTION and PPC_MSG_CALL_FUNC_SINGLE, so the warning
is not needed in that path. The debugger handler handles this case by
simply ignoring IPIs for offline CPUs, so no warning is needed there.
And the reschedule IPI, which is what is occurring in our test environment,
can be safely ignored, so we can simply remove the WARN_ON from xics_ipi_dispatch.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>