ci(release-drafter): remove unused branch config

`branch` is not a valid configuration option for release-drafter,
see
https://github.com/release-drafter/release-drafter#configuration-options

There's is a similar [`references` option][1], but it does nothing when
using GitHub Actions (it's only there for GitHub apps).

There's also `commitish`, but it defaults to the target/branch the
GitHub Action job runs on, so we don't need to set that.

[1]: https://github.com/release-drafter/release-drafter#references

.github/release-drafter.yml

@@ -25,8 +25,6 @@ categories:
 change-template: '- $TITLE (#$NUMBER) @$AUTHOR'
 sort-by: title
 sort-direction: ascending
-branches:
-  - develop
 exclude-labels:
   - 'Skip changelog'
 no-changes-template: 'This release contains minor changes and bugfixes.'