debian/control: remove XS-Python-Version

debian/control: remove XS-Python-Version

Lintian complains:

    W: xen source: python-version-current-is-deprecated XS-Python-Version
    N:
    N:    The use of "current" in the Python-Version or Python3-Version field is
    N:    deprecated.
    N:
    N:    Refer to Debian Python Policy section 3.4 (Specifying Supported
    N:    Versions) for details.
    N:
    N:    Severity: warning
    N:
    N:    Check: languages/python

So, drop it.

We could replace it with X-Python3-Version, but that documentation
says...

    "The optional X-Python3-Version field specifies the versions of
    Python 3 supported. When not specified, it defaults to all currently
    supported Python 3 versions."

...and I think that's fine for the python code included here. Actually,
I'm not sure, but yolo.

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>