Remove source-build-reference-packages source mapping entry

[NikolaMilosavljevic]

After renaming source-build-reference-packages repo to source-build-assets, source-mappings were updated in VMR with dotnet/installer#20839 and dotnet/sdk#53811

We should delete the source-mapping for the old repo and delete the sources from the VMR.

Source-mapping needs to stay until all nodes in the repo graph update their Version.Details.xml entries and remove/rename source-build-reference-packages repo mention.