docs(psci): drop outdated cache maintenance comment

The comment was written when cache maintenance had to be considered when
calling this function. But that argument was dropped a while back and
this comment no longer makes any sense.

Change-Id: Ib68293f23cc3edca3010164dfe8866956b8e1a63
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
diff --git a/lib/psci/psci_common.c b/lib/psci/psci_common.c
index 5418666..3f6b89f 100644
--- a/lib/psci/psci_common.c
+++ b/lib/psci/psci_common.c
@@ -1041,12 +1041,6 @@
 	psci_set_pwr_domains_to_run(end_pwrlvl);
 
 #if ENABLE_PSCI_STAT
-	/*
-	 * Update PSCI stats.
-	 * Caches are off when writing stats data on the power down path.
-	 * Since caches are now enabled, it's necessary to do cache
-	 * maintenance before reading that same data.
-	 */
 	psci_stats_update_pwr_up(end_pwrlvl, &state_info);
 #endif