[kcms/style] Don't call setNeedsSave when style config changes

The style config is saved already when the dialog closes so enabling the KCM apply button doesn't make sense

The comment suggests that this is done for a reason, but the style preview seems to refresh correctly even with this patch

BUG: 439297
2 jobs for !1285 with work/stylechange in 6 minutes and 50 seconds (queued for 8 seconds)
latest merge request