Dropping experimental netfx support

- Set libraries to target netstandard2.1 and updated tests to net5.0. Still learning about the new features. Libraries may also change to net5.0
later if the newer features are useful enough to include directly in the library.
- Removing Platform.cs which was only used by netfx.

Raylib-cs.Tests/Raylib-cs.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net47;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>netcoreapp3.1;net5.0</TargetFrameworks>
     <Platforms>AnyCPU</Platforms>
     <Configurations>Debug;Release</Configurations>
   </PropertyGroup>
@@ -20,6 +20,4 @@
   <ItemGroup>
     <ProjectReference Include="../Raylib-cs/Raylib-cs.csproj" />
   </ItemGroup>
-
-  <Import Project="../netfx.props" />
 </Project>