Remove /system/etc/prop.default

Contents that were in /system/etc/prop.default are now in
/system/build.prop.

The content is placed at the top of build.prop to keep the existing
order between the two files.

Caveat: /prop.default still remains for non-Treble devices.

Bug: 117892318
Test: TH passes
Test: inspect  /system/build.prop and check if it has contents from
the old /system/etc/prop.default file

Change-Id: Ibc74edb88904f953a1e5e6ceef44c461f381c9cf

CleanSpec.mk

@@ -723,6 +723,7 @@ $(call add-clean-step, rm -rf $(HOST_OUT)/vts/*)
 $(call add-clean-step, rm -rf $(HOST_OUT)/framework/vts-tradefed.jar)
 
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/vendor/default.prop)
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/etc/prop.default)
 
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST