This is a follow-up issue of #3649
After refactoring PowerShell assembly load context, we actually don't need Microsoft.PowerShell.CoreCLR.AssemblyLoadContext.dll anymore -- the remaining logic/functionality of this dll should be moved to System.Management.Automation.dll (S.M.A.dll depends on AssemblyLoadContext.dll today).
This might affect DSC because the DSC host is still depending on AssemblyLoadContext.dll, so we should communicate with DSC team on this to make sure nothing breaks.

Tasks

• Remove Microsoft.PowerShell.CoreCLR.AssemblyExtensions.LoadFrom(string assemblyPath) once the PackageManagement module is migrated to .NET Core 2.0.
• Removing Microsoft.PowerShell.CoreCLR.AssemblyLoadContext.dll completely.