[PowerPC] Remove unused IPI type count tracking.

ipi_msg_count is inaccessible outside this file and is never read.

It was introduced in the original SMP support code in r178628 and was never
actually used anywhere.

Remove it to slightly improve IPI performance.

Submitted by: jhibbits

(cherry picked from commit 74f6cb0f316bc0f8fae0b7f31d78d041dc4d509e)