[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
MFC after:	1 week

sys/powerpc/powerpc/mp_machdep.c

@@ -65,7 +65,6 @@ __FBSDID("$FreeBSD$");
 volatile static int ap_awake;
 volatile static u_int ap_letgo;
 volatile static u_quad_t ap_timebase;
-static u_int ipi_msg_cnt[32];
 static struct mtx ap_boot_mtx;
 struct pcb stoppcbs[MAXCPU];
 
@@ -309,7 +308,6 @@ powerpc_ipi_handler(void *arg)
 		return (FILTER_STRAY);
 	while ((msg = ffs(ipimask) - 1) != -1) {
 		ipimask &= ~(1u << msg);
-		ipi_msg_cnt[msg]++;
 		switch (msg) {
 		case IPI_AST:
 			CTR1(KTR_SMP, "%s: IPI_AST", __func__);