Extensions samples should not set SuppressTfmSupportBuildWarnings

https://github.com/dotnet/extensions-samples/blob/c33a8bba6ac4ec61ad19bd4ab9dbd5b5838b6991/Directory.Build.props#L10

The `SuppressTfmSupportBuildWarnings` switch shouldn't be used in samples as it's a workaround to temporarily allow runtimes to consume a package that doesn't support that runtime anymore by falling back to the netstandard2.0 implementation. For System.* packages this switch might not work and we clearly document that this is an unsupported and untested scenario: https://github.com/dotnet/runtime/blob/28a069017673a03473fdd06e8d693a91e3eb9fbb/eng/packaging.targets#L222