Fix flathub ci error

[picsel2]

The flathub buildbot currently fails due to a linter error:

    "errors": [
        "flathub-json-automerge-enabled"
    ],
    "warnings": [
        "module-qtkeychain-buildsystem-is-plain-cmake",
        "module-nextcloud-client-buildsystem-is-plain-cmake"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}

This PR removes the automerge property.

[flathubbot]

Started test build 142683

[flathubbot]

Build 142683 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/125733/com.nextcloud.desktopclient.nextcloud.flatpakref

[tilosp]

Closing since this change was already merged as part of #164