Delete apidiff.mk

Also remove the msg.mk files used for old apicheck.

And export jdiff-doc.zip generated by Soong.

Test: api-stubs-docs-diff
Bug: b/78245848
Change-Id: I329aa375623a7b7a121d58314dab694a326c9f0f
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index f738ab4..97a74ec 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -72,6 +72,7 @@
 LOCAL_DROIDDOC_CUSTOM_ASSET_DIR:=
 LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR:=
 LOCAL_DROIDDOC_DOC_ZIP :=
+LOCAL_DROIDDOC_JDIFF_DOC_ZIP :=
 LOCAL_DROIDDOC_HTML_DIR:=
 LOCAL_DROIDDOC_OPTIONS:=
 LOCAL_DROIDDOC_SOURCE_PATH:=