[Fleet] Remove agent upgrade fix from 8.11.1 release notes (#171200)

## Summary

The 8.11.1 release notes included #170974 which didn't actually land in
8.11.1. We shipped BC2 of 8.11.1 which was built from this Kibana
commit:
https://github.com/elastic/kibana/commits/09feaf416f986b239b8e8ad95ecdda0f9d56ebec.
The PR was not merged until after this commit, so the bug is still
present (though [mitigated
slightly](#169825 (comment)))
in 8.11.1.

This PR removes the erroneous release note from the 8.11.1 release
notes. How can we make sure the fix _does_ get included in the eventual
8.11.2 release notes?

docs/CHANGELOG.asciidoc

@@ -81,7 +81,6 @@ announcement].
 === Bug Fixes
 
 Fleet::
-* Fixes inability to upgrade agents from version 8.10.4 to version 8.11 ({kibana-pull}170974[#170974]).
 * Append space ID to security solution tag ({kibana-pull}170789[#170789]).
 * Modify bulk unenroll to include inactive agents ({kibana-pull}170249[#170249]).
 Lens & Visualizations::