gnu: icecat: Update to 68.3.0-guix0-preview1 [security fixes].

gnu: icecat: Update to 68.3.0-guix0-preview1 [security fixes].

Includes fixes for CVE-2019-11745, CVE-2019-17005, CVE-2019-17008,
CVE-2019-17009, CVE-2019-17010, CVE-2019-17011, and CVE-2019-17012.

* gnu/packages/patches/icecat-gnuzilla-fixes.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove it.
* gnu/packages/gnuzilla.scm (%icecat-version, %icecat-build-id): Update.
(icecat-source): Update hash for the firefox source tarball.  Update to the
latest from gnuzilla.git.  Don't apply icecat-gnuzilla-fixes.patch.  Remove
determinism fix in makeicecat that is now upstream.  Tweak a status message.
(icecat)[arguments]: Add "--with-unsigned-addon-scopes=app" configure flag.
* gnu/packages/patches/icecat-makeicecat.patch: Adapt.