Don't invoke prepare-pipelines for CI convention for js, python

[scbedd]

An alternative to implementing it this way is to make the disallowed repositories arguments to the tool. EG:

--disallowed "azure/azure-sdk-for-js,azure/azure-sdk-for-python"

And then we split this input, and use it the same way that we're using the hardcoded values. Figured I'd do the extremely easy first, and prompt opinions from @weshaggard and @benbp based on this simple version.