[9.18] fix: doc: Remove 9.21-only release note from May 2026 release notes

Issue #5826 has two different fixes: one released in April 2026 that
applies to 9.20 and 9.18, and another released in May 2026 that applies
to 9.21. The 9.21 release note was mistakenly included in the release
notes for 9.20 and 9.18. This commit removes it.

Backport of MR !12067

Merge branch 'backport-andoni/fix-doc-duplicate-note-5826-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12078

doc/notes/notes-9.18.49.rst

@@ -101,13 +101,6 @@ Bug Fixes
   handled consistently with RRSIG records during update processing.
   :gl:`#5818`
 
-- Fix :option:`rndc modzone` behavior for a zone in named.conf.
-
-  If a zone was present in the configuration file and not originally
-  added by :option:`rndc addzone`, :option:`rndc modzone` for that zone would succeed
-  once but subsequent :option:`rndc modzone` attempts would fail. This has been
-  fixed. :gl:`#5826`
-
 - Fix zone verification of NSEC3 signed zones.
 
   Previously, when computing the compressed bitmap during verification