Revert "Expose Marshall.LoadLibrary API" (#34322)

In API review: https://github.com/dotnet/corefx/issues/32015,

The LoadLibrary APIs were originally Approved to live in
System.Runtime.InteropServices.Marshal class.
https://github.com/dotnet/corefx/issues/32015#issuecomment-428775858

However, recently the decision was changed such that the APIs are in a new NativeLibrary class.
https://github.com/dotnet/corefx/issues/32015#issuecomment-448324606

Therefore, undoing the changes to Marshal API.
I'll submit another PR for System.Runtime.InteropServices.NativeLibrary
once the corresponding change in CoreCLR is checked in.

This reverts commit 859351c.

Author: swaroop-sridhar

src/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.csproj

@@ -9,7 +9,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Runtime.InteropServices.cs" />
-    <Compile Condition="'$(TargetsNetCoreApp)' == 'true'" Include="System.Runtime.InteropServices.netcoreapp.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\System.Reflection\ref\System.Reflection.csproj" />