remove legacy projects and package spec's from solution about songhaycore

[BryanWilhite]

i am increasingly confident that the combination of <TargetFrameworks> and conditional compilation is all that is needed to build from one .NET Standard project, supporting multiple platforms

https://docs.microsoft.com/en-us/dotnet/standard/frameworks

proposed support for this studio:

target framework moniker (TFM)	preprocessor symbol
netcoreapp2.2	NETCOREAPP2_2
netcoreapp2.1	NETCOREAPP2_1
netstandard2.0	NETSTANDARD2_0
net462	NET462
net452	NET452

my confidence is shaky around a scenario where, say, support for net35 is needed because netstandard2.0 supports more than what is in net35 and i am not going to manually tree-shake out what netstandard2.0 supports with a bunch of #if-#elif blocks ( 🔬 one could test how this scenario works out by adding net35 to the project and reference its build in a .NET 3.5 test project)

the intention here is to use #if-#elif only to block out what netstandard2.0 does not support—and this support should eliminate the need for #19 and #2

[BryanWilhite]

this @RickStrahl post is very important: https://weblog.west-wind.com/posts/2017/Sep/18/Conditional-TargetFrameworks-for-MultiTargeted-NET-SDK-Projects-on-CrossPlatform-Builds

[BryanWilhite]

also i have been missing this: https://docs.microsoft.com/en-us/dotnet/core/tutorials/libraries#how-to-multitarget

[BryanWilhite]

more @RickStrahl: https://weblog.west-wind.com/posts/2017/jun/22/multitargeting-and-porting-a-net-library-to-net-core-20

[BryanWilhite]

this issue addressed Error NU5125: The 'licenseUrl' element will be deprecated [ https://github.com/NuGet/Home/issues/7509 ], replacing licenseUrl with the license element and a files entry: https://docs.microsoft.com/en-us/nuget/reference/nuspec#license

[BryanWilhite]

also i should apologize to me for not using a branch during the resolution of this issue; pushing code that will not build to the master branch was not confidence building for any visitor that might have stumbled upon this repo 🙄