Remove exported2_system_prop

It's not used anymore.

Bug: 161659925
Test: boot
Change-Id: I5b08bdace28a509d464759a66025c951178225c6
Merged-In: I5b08bdace28a509d464759a66025c951178225c6
(cherry picked from commit 7d96ddbfb0b2e34c79b921538bebcaaf35c988ff)
diff --git a/private/compat/30.0/30.0.cil b/private/compat/30.0/30.0.cil
index e13c289..44d7535 100644
--- a/private/compat/30.0/30.0.cil
+++ b/private/compat/30.0/30.0.cil
@@ -10,6 +10,7 @@
 (type exported2_config_prop)
 (type exported2_default_prop)
 (type exported2_radio_prop)
+(type exported2_system_prop)
 (type exported2_vold_prop)
 (type exported3_default_prop)
 (type exported3_radio_prop)