[Darc/Maestro++] Remove insertion of InitialAssetLocation

Whenever we insert a build into BAR we add an initial asset location to all assets of that build. This initial asset location is (usually) just a link to an AzDO URL that can be used to obtain a JSON string with the names of the artifacts produced by that build. We can/should remove that, because:

- This initial asset location was never useful.
- If necessary we can reconstruct that URL from the other properties of the Build entity
- Darc gather-drop doesn't know how to handle that URL format
- It will simplify things.

Here is the links to the codes that do the insertion:

- https://github.com/dotnet/arcade-services/blob/master/src/Maestro/Microsoft.DotNet.Maestro.Tasks/src/PushMetadataToBuildAssetRegistry.cs#L274
- https://github.com/dotnet/arcade-services/blob/master/src/Maestro/Microsoft.DotNet.Maestro.Tasks/src/PushMetadataToBuildAssetRegistry.cs#L293

/cc @riarenas @epananth