Simplify PubsubIO projectId management

PubsubIO uses some (probably) legacy code around projectId management, which among several other suboptimalities causes pipelines without GcpOptions.project specified to fail. This is not necessary, because projectId is already contained in projects/<project>/topics/<topic> when reading specified topic. The projectId should be read from there and (possibly to maintain backwards compatibility) be overridden by GcpOptions.