Remove option "-J" from cmdline docs

[vasily-v-ryabov]

Documentation

File Doc/using/cmdline.rst mentions Options you shouldn’t use:

Options you shouldn't use
~~~~~~~~~~~~~~~~~~~~~~~~~

.. option:: -J

   Reserved for use by Jython_.

.. _Jython: https://www.jython.org/

Option -J was added in Python 2.6. Since all Jython related things are being removed, I'd suggest to remove reference to this option from the docs as well.

[sobolevn]

I support this idea.

[StanFromIreland]

Should we not deprecate it (similarly to this), it currently serves no purpose and 3.X is not supported by Jython? Jython3 seems to have halted (8 months of inactivity).