Remove the notion of a Fake subscription from PubsubIO

[kennknowles]

PubsubSubscription should be cleaned up and references to a "Fake" type removed.

start_signal/ is no longer needed since StreamingCreate has been replaced with an UnboundedReadFromBoundedSource.
random/ was created but never used in Dataflow or Apache Beam

Imported from Jira BEAM-496. Original Jira may contain additional context.
Reported by: lcwik.