PipelineTranslationTest.testProtoAgainstRehydrated() over-validates translated proto structure

PipelineTranslationTest.testProtoAgainstRehydrated() validates round-tripping of a Pipeline object to and form proto, verifying each of the rehydrated fields. From conversations with [~lcwik], the validation logic is over-specified. There is no requirement that equivalent objects are serialized using the same ID. So iterating to find the set of unique IDs and validating the count is incorrect.

Imported from Jira [BEAM-4284](https://issues.apache.org/jira/browse/BEAM-4284). Original Jira may contain additional context.
Reported by: swegner.