Remove unnecessary package references. (#2990)

* Remove references to .NET Standard 1.x packages.
* Also remove `System.Bufffers`.

ICSharpCode.ILSpyX/ICSharpCode.ILSpyX.csproj

@@ -64,14 +64,11 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.IO.Compression" Version="4.3.0" />
     <PackageReference Include="System.Reflection.Metadata" Version="$(SystemReflectionMetadataVersion)" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="$(SystemCompilerServicesUnsafeVersion)" />
     <PackageReference Include="System.Composition" Version="$(SystemCompositionVersion)" />
-    <PackageReference Include="System.Runtime.Loader" Version="4.3.0" />
     <PackageReference Include="Mono.Cecil" Version="$(MonoCecilVersion)" />
     <PackageReference Include="K4os.Compression.LZ4" Version="1.3.5" />
-    <PackageReference Include="System.Buffers" Version="4.5.1" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>