bug#29859: 25.3; MozRepl support is/will be no longer useful

[Tim Landscheidt]

Firefox 57 ("Quantum") removed the possibility to install
extensions à la MozRepl
(https://addons.mozilla.org/de/firefox/addon/mozrepl/).  To
quote MozRepl's README.md (https://github.com/bard/mozrepl):

| Important Notice

| Key technologies upon which MozRepl depends will be retired
| from the Mozilla platform in November 2017.  If you are re-
| lying on MozRepl, please investigate migration paths.  The
| last known compatible version is Firefox 54.

| […]

The current Firefox ESR release is based on Firefox 52 and
should allow MozRepl to be installed until August 2018
(https://www.mozilla.org/en-US/firefox/organizations/), but
I did not test that.

Thus the large code section in lisp/progmodes/js.el relating
to MozRepl is unfortunately no longer useful and could be
removed now or in September 2018 at the latest.  (Personal
guess: Nobody is using Emacs with Firefox ESR, so removing
it from the next release would not hurt anyone.)

(AFAIUI, the closest match to MozRepl's functionality is now
Marionette
(https://firefox-source-docs.mozilla.org/testing/marionette/marionette/)
which implements a WebDriver interface (?); supporting that
would be cool and more general, but also probably way more
complicated than MozRepl.)