Removed code to obtain and use file size.

It is useful to know when the file size is smaller than the buffer size, so that
we do not unnecessarily allocate a second buffer to prefetch. But obtaining the
file size comes with overhead, and complicates the interface since we want to
use `forward_buffer` to work for write buffers as well. It does not make sense
to disable prefetching based on the file size for write buffers.
Implemented forward_write_buffer.