[3.0.x] Refs #28954 -- Remove remaining Jython documentation.

There is still no further progress on Jython 3 since support for Jython
was stripped in Django 2.0. The developers experimenting with Jython 3
have stated that they would need to start from scratch, so it doesn't
look as though it will materialize for years to come.

Python 2 and Django 1.11, the last version to support Python 2.7, will
both be end-of-life before Django 3.1 is released. We should no longer
instruct users to use an unsupported version of Django.

Backport of d8b6a76bc7 from master

docs/howto/index.txt

@@ -20,7 +20,6 @@ you quickly accomplish common tasks.
    upgrade-version
    error-reporting
    initial-data
-   jython
    legacy-databases
    outputting-csv
    outputting-pdf

docs/howto/jython.txt

@@ -1,14 +0,0 @@
-================
-Django on Jython
-================
-
-.. index:: Jython, Java, JVM
-
-Jython_ is an implementation of Python that runs on the Java platform (JVM).
-
-.. _jython: http://www.jython.org/
-
-If you want to use Jython (which supports only Python 2.7 at the time of this
-writing), you must use Django 1.11.x (the last series to support Python 2).
-Jython support may be readded to Django if a Python 3 compatible Jython is
-released.

docs/topics/install.txt

@@ -13,13 +13,6 @@ details.
 Get the latest version of Python at https://www.python.org/downloads/ or with
 your operating system's package manager.
 
-.. admonition:: Django on Jython
-
-    Jython_ (a Python implementation for the Java platform) is not compatible
-    with Python 3, so Django ≥ 2.0 cannot run on Jython.
-
-.. _jython: http://www.jython.org/
-
 .. admonition:: Python on Windows
 
     If you are just starting with Django and using Windows, you may find