Stop running license scan pipeline on preview branches after release

The license scan pipeline ran for the 10.0.1xx-preview6 branch today: https://dev.azure.com/dnceng/internal/_build/results?buildId=2754461&view=results. This is because the pipeline is scheduled to run Mondays at 7am UTC. However, preview 6 was released last week, so the pipeline should no longer run on this branch. Similarly, because there were failures in the pipeline, [a PR targeting the preview6 branch](https://github.com/dotnet/dotnet/pull/1549) was created. This PR is not valid because we already released preview6.