CLEANUP: fd: remove unused cb->b pointers in the struct fdtab

CLEANUP: fd: remove unused cb->b pointers in the struct fdtab

These pointers were used to hold pointers to buffers in the past, but
since we introduced the stream interface, they're no longer used but
they were still sometimes set.

Removing them shrink the struct fdtab from 32 to 24 bytes on 32-bit machines,
and from 52 to 36 bytes on 64-bit machines, which is a significant saving. A
quick tests shows a steady 0.5% performance gain, probably due to the better
cache efficiency.