Remove unnecessary LOCAL_BUILT_MODULE_STEM

'/' is no longer allowed in module names or stems, and isn't necessary
here.

Test: m brillo-update-payload-key
Change-Id: I44a1f693775b099ce3a4438cf8ea195b92e08f8b
diff --git a/Android.mk b/Android.mk
index 1beada9..80a8afd 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1038,7 +1038,6 @@
 LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/update_engine
 LOCAL_MODULE_STEM := update-payload-key.pub.pem
 LOCAL_SRC_FILES := update_payload_key/brillo-update-payload-key.pub.pem
-LOCAL_BUILT_MODULE_STEM := update_payload_key/brillo-update-payload-key.pub.pem
 include $(BUILD_PREBUILT)
 endif  # PRODUCT_IOT