Remove SourceLink SDK references

[ericstj]

@ViktorHofer let me know that we no longer need to have explicit SDK references to these source-link packages.

I'm removing them and also testing the behavior of build to ensure we still have source-link info in our binaries per https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/sourcelink.

[ViktorHofer]

Please also remove the other hits, i.e. Tools.props: https://github.com/search?q=repo%3Adotnet%2Fmachinelearning%20sourcelink&type=code

[ericstj]

I found quite a few other dead dependencies here that I was able to clean up. I have also confirmed that the binaries / pdbs / packages still have source link info after this change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.79%. Comparing base (f22b60a) to head (bbc8bd8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7037   +/-   ##
=======================================
  Coverage   68.79%   68.79%           
=======================================
  Files        1254     1254           
  Lines      250204   250204           
  Branches    25529    25529           
=======================================
+ Hits       172129   172134    +5     
+ Misses      71465    71460    -5     
  Partials     6610     6610           

Flag	Coverage Δ
Debug	68.79% <ø> (+<0.01%)	⬆️
production	63.23% <ø> (+<0.01%)	⬆️
test	88.50% <ø> (ø)

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see 2 files with indirect coverage changes