Replace RootAllApplicationAssemblies with TrimMode (#6327)

CoreRT aligned trimming settings with how official .NET 5 does trimming.

frameworks/CSharp/aspnetcore-corert/PlatformBenchmarks/PlatformBenchmarks.csproj

@@ -6,8 +6,7 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
 
     <!-- Opt out of the "easy mode" of the CoreRT compiler (http://aka.ms/OptimizeCoreRT) -->
-    <RootAllApplicationAssemblies>false</RootAllApplicationAssemblies>
-    <IlcGenerateCompleteTypeMetadata>false</IlcGenerateCompleteTypeMetadata>
+    <TrimMode>link</TrimMode>
     <IlcGenerateStackTraceData>false</IlcGenerateStackTraceData>
   </PropertyGroup>