gnu: icecat: Update to 68.2.0-guix0-preview2.

gnu: icecat: Update to 68.2.0-guix0-preview2.

* gnu/packages/gnuzilla.scm (%icecat-version): Update.
(%icecat-build-id): New variable.
(icecat-source): Update gnuzilla repo commit and hash.
(icecat)[arguments]: In the custom 'configure' phase, set the MOZ_BUILD_DATE
environment variable to the value of %icecat-build-id.
* gnu/packages/patches/icecat-gnuzilla-fixes.patch: Remove changes that
are now in the upstream repository.  Add more pending changes, including
disabling the MOZ_SERVICES_HEALTHREPORT and MOZ_BLOCK_PROFILE_DOWNGRADE
build flags, fixing a problem that prevented MOZ_DATA_REPORTING
from being disabled, and fixes to the branding.
* gnu/packages/patches/icecat-makeicecat.patch: Adapt to upstream changes,
and changes in icecat-gnuzilla-fixes.patch.  Remove a hunk that disabled
rewrites to aboutRights.dtd in the l10n directory.