Delete explicit SourceLink reference, which is in the SDK

[Smaug123]

Per https://github.com/dotnet/sourcelink :

Starting with .NET 8, Source Link for the following source control providers is included in the .NET SDK and enabled by default: GitHub or GitHub Enterprise

Dependabot has started upgrading this package, which should be completely unnecessary anyway.

Supersedes #51

[Smaug123]

I checked the DLL using https://github.com/Smaug123/dotnet-pdb-analysis/ and it contains SourceLink metadata (observe the 404s, indicating that it tried to download the source for a generated file):

==== Examining file lib/netstandard2.0/HeterogeneousCollections.dll ====
Source linked obj dir (dubious check but it's what NuGetPackageExplorer does): /_//HeterogeneousCollections/obj/Release/netstandard2.0/.NETStandard,Version=v2.0.AssemblyAttributes.fs to https://raw.githubusercontent.com/G-Research/HeterogeneousCollections/97504f0589592c900a87d45fd695bf4b7c820c49//HeterogeneousCollections/obj/Release/netstandard2.0/.NETStandard,Version=v2.0.AssemblyAttributes.fs
Source linked to non-existent target: /_//HeterogeneousCollections/obj/Release/netstandard2.0/.NETStandard,Version=v2.0.AssemblyAttributes.fs to https://raw.githubusercontent.com/G-Research/HeterogeneousCollections/97504f0589592c900a87d45fd695bf4b7c820c49//HeterogeneousCollections/obj/Release/netstandard2.0/.NETStandard,Version=v2.0.AssemblyAttributes.fs, error response 404: Not Found