Drop now unnecessary editing of xmllint command line in CI tests (!24)

GNOME 3's yelp doesn't use scrollkeeper or the OMF catalog, so the
constructed Makefile doesn't use xmllint to validate the scrollkeeper
DTD file.  Therefore remove attempted sed edit of that line which no
longer exists in the Makefile.

Note that help/Makefile.am's @YELP_HELP_RULES@ automake macro expansion
comes from /usr/share/aclocal/yelp.m4 [1].

Commit which previously needed to add the sed edit:
    cbb25a2511
    Stop xmllint scrollkeeper-omf.dtd fetch failure breaking CI tests (#9)

[1] Yelp > Yelp Tools > yelp.m4
    http://yelp.io/tools/yelp.m4.html

Closes !24 - Port to GNOME 3 yelp-tools documentation infrastructure

.gitlab-ci.yml

@@ -35,8 +35,6 @@ stages:
     - ./autogen.sh
     - nproc=`grep -c '^processor' /proc/cpuinfo` || nproc=1
     - echo nproc=$nproc
-    # Prevent xmllint fetching scrollkeeper DTD file remotely.
-    - sed -i '/xmllint.*http.*scrollkeeper-omf.dtd/s|xmllint |xmllint --nonet |' help/Makefile
     - make -j $nproc
     # Exclude specific unit test which fails without /dev/disk in Docker images.
     - export GTEST_FILTER='-BlockSpecialTest.NamedBlockSpecialObjectBySymlinkMatches'