chore: remove tarantool-sys submodule branch

Follow-up for 2b1bd622

> submodule.<name>.branch
>
> A remote branch name for tracking updates in the upstream submodule.
> If the option is not specified, it defaults to the remote HEAD.

The `origin/HEAD` is the default branch in GitLab. In case of
tarantool-sys nowadays it corresponds to `2.11.0-picodata`.

This option is responsible for the correct operation of

    git submodule update --remote tarantool-sys