gnu: icecat: Remove the bundled Onion Browser Button [security].

gnu: icecat: Remove the bundled Onion Browser Button [security].

Fixes <https://bugs.gnu.org/35728>.

The Onion Browser Button (tortm-browser-button@jeremybenthum) version 0.1.8,
available from addons.mozilla.org and bundled with IceCat, malfunctions with
both IceCat 68.9 and Firefox ESR 68.9.  It tells the user that it's connected
to Tor, but fails to route traffic through it.  The same happens on Debian 9
with its "firefox-esr" package.

Remove this extension from IceCat, pending further investigation.

* gnu/packages/gnuzilla.scm (icecat-source): Remove the "data/extensions/
tortm-browser-button@jeremybenthum" directory before running makeicecat.