Remove dontaudit of system_internal_prop

system_inernal_prop is meant to be used only within system/, so vendor
specific sepolicy shouldn't touch system_internal_prop.

Exempt-From-Owner-Approval: cherry-pick

Bug: 150331497
Test: system/sepolicy/tools/build_policies.sh
Change-Id: Idf3b92c1058a8075e0479e7efdd3b6419e7abfa5
Merged-In: Idf3b92c1058a8075e0479e7efdd3b6419e7abfa5
(cherry picked from commit 4e3894abf0d4d4daea5f1bbf35b4c339bc3948d3)