log: remove unnecessary stubs

log: remove unnecessary stubs

The read/seek/close stub functions are unnecessary on the
log stream.  Per glibc fopencookie man page:

       cookie_read_function_t *read
              If *read is a null pointer, then reads from  the  custom  stream
              always return end of file.

       cookie_seek_function_t *seek
              If *seek is a null pointer, then it is not possible  to  perform
              seek operations on the stream.

       cookie_close_function_t *close
              If  *close is NULL, then no special action is performed when the
              stream is closed.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>