Fix release notes generation script

Script was accidentally trying to import `common.sh`.

Closes gh-15349

Author: philwebb

ci/scripts/generate-release-notes.sh

@@ -1,8 +1,6 @@
 #!/bin/bash
 set -e
 
-source $(dirname $0)/common.sh
-
 version=$( cat version/version )
 
 milestone=${version}