Remove "Past versions" section from release notes

This came in via matplotlib#21251 and I vaguely remember it had
a purpose (possibly something with navigation / what
is rendered in the sidebars?) However, in the
current docs, it looks rather useless.
https://matplotlib.org/devdocs/users/release_notes.html

Author: timhoffm

doc/users/release_notes.rst

@@ -75,10 +75,6 @@ Version 3.4
     prev_whats_new/github_stats_3.4.1.rst
     prev_whats_new/github_stats_3.4.0.rst
 
-=============
-Past versions
-=============
-
 Version 3.3
 ^^^^^^^^^^^
 .. toctree::