Remove compatibility testing related code from KafkaIO.Read after SDF implementation has fully replaced the legacy read

There were properties/features in KafkaIO.Read that were only available in Legacy or SDF read. In order to avoid hiding the lack of support a solution was implemented in: https://github.com/apache/beam/pull/16909 Further details can be found in the PR description about it.

When SDF support/compatibility reaches the expected levels, and the SDF read completely replaces the legacy read this system should be cleaned up as well.