gnu: Remove icecat-68 source.

It was necessary for icedove@68 which is now based on icecat@78.

* gnu/packages/gnuzilla.scm (mozilla-68-compare-locales): Remove.
(all-mozilla-68-locales): Remove.
(%icecat-68-version): Remove.
(icecat-68-source): Remove.
* gnu/packages/patches/icecat-68-makeicecat.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove it.

Modified-By: Mark H Weaver <mhw@netris.org>
Signed-off-by: Mark H Weaver <mhw@netris.org>