physfs: Remove size property

[Unrud]

Fixes the flatpak-builder-lint error:

{
    "errors": [
        "manifest-unknown-properties"
    ],
    "info": [
        "manifest-unknown-properties: ['size']"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}