stream: remove unused read_chunk field

It was set, but its value was never used. The stream cache used to use
it, but it was removed. It controlled how much data it tried to read
from the underlying stream at once.

The user can now control the buffer size with --stream-buffer-size,
which achieves a similar effect, because the stream will in the common
case read half of the buffer size at once. In fact, the new default size
is 128KB, i.e. 64KB read size, which is as much as stream_file and
stream_cb requested by default. stream_memory requested more, but it
doesn't matter anyway. Only stream_smb set a larger size with 128KB.