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/tasks/apidiff.mk b/core/tasks/apidiff.mk
index 4eb59af..76e4749 100644
--- a/core/tasks/apidiff.mk
+++ b/core/tasks/apidiff.mk
@@ -18,4 +18,4 @@
 
 .PHONY: api-diff
 
-api-diff: offline-sdk-referenceonly-diff
+api-diff: api-stubs-docs-jdiff