Build fails on VS Enterprise 2019 Version 16.4.5

[pragyamehta]

**The TargetFramework value 'xamarinios10' was not recognized. It may be misspelled. If not, then the TargetFrameworkIdentifier and/or TargetFrameworkVersion properties must be specified explicitly.

The TargetFramework value 'monoandroid81' was not recognized. It may be misspelled. If not, then the TargetFrameworkIdentifier and/or TargetFrameworkVersion properties must be specified explicitly.**

Mobile development with .NET is already installed for this VS

[brendandburns]

I admit I don't really use VS Enterprise so I'm not sure I can help with this. If you figure it out, I'm happy to take a PR to fix the .sln/.csproj files.

[tg123]

you may want use dotnet build to avoid this
@brendanburns I suggest to remove those targets as in #345 they are never in our nuget package and covered by netstandard

the reason they were introduce is to support ssl and websocket on those platfrom

[brendandburns]

@tg123 does that mean that a Xamarin user can use the netstandard nugets from mobile?

[tg123]

@brendanburns
Yes
https://docs.microsoft.com/en-us/xamarin/cross-platform/app-fundamentals/net-standard
https://docs.microsoft.com/en-us/dotnet/standard/net-standard

I am happy to reforge the targets if you approve

[brendandburns]

@tg123 sure, that sounds great, thanks.