docs: remove default_branch from config.yaml

By setting the `default_branch` the generation of release notes fails to
capture all of the tags releases within a given branch.

Author: jackhodgkiss

releasenotes/config.yaml

@@ -2,4 +2,3 @@
 # NOTE(upgrade): This needs to be updated to the current release's major version.
 release_tag_re: stackhpc/18\.\d+\.\d+\.\d
 ignore_null_merges: false
-default_branch: 'stackhpc/2025.1'