8.15 RN file cleanup in 8.x branch (#6377)

Author: nastasha-solomon

docs/release-notes/8.15.asciidoc

@@ -21,10 +21,6 @@
 [[release-notes-8.15.4]]
 === 8.15.4
 
-[discrete]
-[[known-issue-8.15.4]]
-==== Known issues
-
 [discrete]
 [[enhancements-8.15.4]]
 ==== Enhancements
@@ -47,10 +43,6 @@
 [[release-notes-8.15.3]]
 === 8.15.3
 
-[discrete]
-[[known-issue-8.15.3]]
-==== Known issues
-
 [discrete]
 [[bug-fixes-8.15.3]]
 ==== Bug fixes