Don't create submanifets

Sorry, I wasn't aware of the following requirements. We can fix it with the next release.

> **Creating submanifests**
>
> Sub-manifests with sources must only be created when they are generated using an external tooling or in an automatic manner, otherwise, everything must be in the main manifest.

Source: https://docs.flathub.org/docs/for-app-authors/requirements#flatpak-builder-manifest-style-guide

_Originally posted by @yakushabb in https://github.com/flathub/io.github.ihhub.Fheroes2/issues/66#issuecomment-4529250235_

No file or test is named. Read the linked Flathub manifest style guide, then search the repository for submanifest references; the work is complete when sources are kept in the main manifest unless the submanifest is generated externally or automatically.

Written by the indexing model from the issue text.