Remove SignNupkg and SignSDKResolverContents targets

[livarcocc]

We have changed our sign project to sign all assemblies together, however, we had to keep the targets above around so as not to break build definitions that still invoke them.

Once this change permeates to other branches, we can then remove the steps from the build defs and delete these empty targets.

See PR dotnet/cli#8601

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[github-actions]

This issue will now be closed since it has been labeled 'stale' without activity for 30 days.