Remove vss-tools dependency to VSS

As of today we have a sort of circular dependency

VSS repo defines syntax (in *.md files)
VSS-tools depends on syntax
But then also VSS depends on VSS-tools to check that the standard catalog follows specified syntax.
And vss-tools depends on vss for CI.

Would it be an idea to try to break up this dependency? My idea - for now - is to remove CI dependency towards VSS repo.

In short:

Let VSS-tools CI (https://github.com/COVESA/vss-tools/blob/master/.github/workflows/buildcheck.yml) be independent from VSS. I.e. do not check out VSS. Instead rely only on test cases in https://github.com/COVESA/vss-tools/tree/master/tests/vspec

I.e. remove the following lines (and some more) from vss-tools CI:

     - name: Test mandatory targets
        run: |
          cd ${{ env.VSS_TOOLS_PATH }}
          pipenv run make -C "${{ env.VSS_PATH }}" travis_targets
      - name: Test optional targets.
        run: |
          cd ${{ env.VSS_TOOLS_PATH }}
          pipenv run make -C "${{ env.VSS_PATH }}" -k travis_optional ||
            echo "::warning title=travis_optional::optional targets FAILED"

This would simplify cases where we today need to change Makefile as the repos then typically get out of synch with temporary build errors until both repos are updated . We might need/want to move some parts to vss-tools at the same time, like how to build binary tools.

This is a little related to #121.

COVESA / vss-tools

Remove vss-tools dependency to VSS #288