Revert Xamarin placeholders for System.Memory package

[marek-safar]

Please revert dotnet/corefx#28034 change, we couldn't get our bits working properly on time for dev15-8 release.

@weshaggard this needs to happen for 4.5 version of System.Memory which I guess is 2.1 release. The fix can stay in master as we'll need it for future versions.

Note: that'll require a better way to coordinate the supported nuget package versions for Xamarin.

It should also address few related issues like aspnet/SignalR#1886

[weshaggard]

@marek-safar if we do that change will the netstandard asset in the package work correctly on Xamarin? Hopefully it will but I think it is worth validating before we do this change.

[marek-safar]

If they work on .net desktop they should

[ahsonkhan]

cc @joperezr

[tarekgh]

Should be fixed by dotnet/corefx#29925 in release/2.1 branch. Part of 2.1.1 release.