gs: use default block size

Solves: #846 (closed)

Instead of using download_buffer_size() from gcs::ClientOptions, a default blockSize value should be used. The value is set to 3 * 1024 * 1024 / 2, as this is the result from https://github.com/googleapis/google-cloud-cpp/issues/2657 (which is also used in https://github.com/googleapis/google-cloud-cpp/blob/b0fb7d7043df69c9a7059378189d1ebab6b479cb/google/cloud/storage/client_options.cc#L68). More details can be found here: https://github.com/googleapis/google-cloud-cpp/issues/7505.

Let me know if we want anything different.