Remove references to nonplat sepolicy

"nonplat" was renamed to "vendor" in Android Pie, but was retained
here for Treble compatibility.

We're now outside of the compatbility window for these devices so
it can safely be removed.

Test: atest treble_sepolicy_tests
Change-Id: Iaa22af41a07b13adb7290f570db7a9d43b6e85cc
diff --git a/private/compat/31.0/31.0.cil b/private/compat/31.0/31.0.cil
index 35059a9..84d70c2 100644
--- a/private/compat/31.0/31.0.cil
+++ b/private/compat/31.0/31.0.cil
@@ -4,6 +4,7 @@
 (type apex_scheduling_data_file)
 (type apex_wifi_data_file)
 (type healthd_exec)
+(type nonplat_service_contexts_file)
 (type vr_hwc)
 (type vr_hwc_exec)