Disables mgmt build on sdk/ folder and disables old azure-sdk-for-net…

[mitchdenny]

This PR finalizes the transition to the shorter running PR validation pipelines that have been setup. Basically once this merges people who are working exclusively in the sdk/ folder won't need to worry about waiting for the azure-sdk-for-net - mgmt or azure-sdk-for-net - client pipelines to run.

[mitchdenny]

I've disabled the CI and PR triggers in the defintiion in Azure DevOps so it won't fire anymore and I've removed the client.ymll file in this PR. Once its merged I'll hold off deleting the old pipeline definition for a while since once we delete it all the history is gone. But we can clean it up in a week or so (unless you don't think that there is any value in keeping it around for a bit longer).

[weshaggard]

But we can clean it up in a week or so (unless you don't think that there is any value in keeping it around for a bit longer).

For the one in the public team there isn't any real value in keeping it around. We still have the equivalent "net - client" in internal.