Finalize release notes for v8.15.4

Author: elasticsearchmachine

docs/reference/release-notes/8.15.4.asciidoc

@@ -1,8 +1,6 @@
 [[release-notes-8.15.4]]
 == {es} version 8.15.4
 
-coming[8.15.4]
-
 Also see <<breaking-changes-8.15,Breaking changes in 8.15>>.
 
 [[bug-8.15.4]]