gnu: icecat: Update to 68.10.0-guix0-preview1 [security-fixes].

Includes fixes for CVE-2020-12417, CVE-2020-12418, CVE-2020-12419,
CVE-2020-12420, and CVE-2020-12421.

* gnu/packages/gnuzilla.scm (%icecat-version, %icecat-build-id): Update.
(icecat-source): Update gnuzilla commit, base version, and hashes.  Remove
code that deleted the Onion Browser Button extension, which is no longer
bundled upstream.
* gnu/packages/patches/icecat-makeicecat.patch: Adapt to new version.