Do not initialize BUILD_BROKEN_TREBLE_SYSPROP_NEVERALLOW

BUILD_BROKEN_TREBLE_SYSPROP_NEVERALLOW is meant to be set depending on
targets, not devices. This allows that value to be set outside
BoardConfig.mk.

Bug: 131162102
Bug: 142684203
Test: m sepolicy_tests
Change-Id: I14f7cd06dcbaf1b5354c648079a815d7b6cc6f3a