[libc++][doc] Removes LLVM 19 Release Notes. (#134894)

There will be no more LLVM-19 releases so we will not backport patches
for this release. This makes these Release Notes obsolete.

Author: mordante

libcxx/docs/ReleaseNotes.rst

@@ -5,7 +5,6 @@
     :hidden:
 
     ReleaseNotes/20
-    ReleaseNotes/19
 
 .. The release notes are in versioned files, but we make sure to keep publishing
 .. them in an unversioned ReleaseNotes.html page for external sites to reference.