github actions: Drop HTML validation

Its seems `xmllint` is mainly an XML validator and not a HTML validator
and can't find a decent command line HTML validator.

.github/workflows/action.yml

@@ -145,9 +145,3 @@ jobs:
           cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON ${{ matrix.configure-opts }} -DCMAKE_FIND_FRAMEWORK=NEVER
           cmake --build .
           ctest -V
-
-      - name: Check documentation
-        if: startsWith(matrix.os,'ubuntu') && startsWith(matrix.build-system,'autotools')
-        run: |
-          xmllint --valid --noout doc/html/*.html;
-          xmllint --valid --noout doc/html/api/*.html;