set_bb_env: include DEBUG_BUILD in unset_bb_env

If DEBUG_BUILD isn't unset the subsequent flavor perf/user carry debug artefacts

Change-Id: Ia4ebe9a2f64f4ea76311b3ef91f42c2941d3a934

set_bb_env.sh

@@ -737,7 +737,7 @@ rebake() {
 }
 
 unset_bb_env() {
-  unset DISTRO MACHINE VARIANT
+  unset DISTRO MACHINE VARIANT DEBUG_BUILD
 }
 
 # Find build templates from qti meta layer.