Don't set IsTrimmable on netcoreapp in 6.0

These assemblies now opt into trimming using AssemblyMetadata:
dotnet/runtime#48428.

The SDK side of the change was made in dotnet/sdk#16094.

For more context, see
https://github.com/mono/linker/blob/main/docs/design/trimmed-assemblies.md#net-6.

src/redist/targets/GenerateBundledVersions.targets

@@ -250,7 +250,6 @@ Copyright (c) .NET Foundation. All rights reserved.
                               TargetingPackVersion="$(MicrosoftNETCoreAppRefPackageVersion)"
                               RuntimePackNamePatterns="Microsoft.NETCore.App.Runtime.**RID**"
                               RuntimePackRuntimeIdentifiers="@(NetCoreRuntimePackRids, '%3B')"
-                              IsTrimmable="true"
                               />
 
     <KnownAppHostPack Include="Microsoft.NETCore.App"