Remove the branch name

[tilosp]

This way buildbot sets the branch name to beta or stable depending on the git branch.
Once this is merged i would like to rebase the beta branch onto master to keep the difference minimal.

Closes: #30

[flathubbot]

Started test build 53

[flathubbot]

Build 53 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/50/org.nextcloud.Nextcloud.flatpakref

[tilosp]

Closed in favor of #33