docs: renice(1): Remove obsolete BUGS text

docs: renice(1): Remove obsolete BUGS text

Already at least as far back as util-linux 2.2, renice uses
getpriority(2) to fetch the process's old nice value. Thus,
the "problem" discussed in this BUGS note disappeared long ago.
This is trivially demonstrable:

    $ sleep 100 &
    [1] 24322
    $ renice -n 5 24322
    24322 (process ID) old priority 0, new priority 5
    $ renice -n 10 24322
    24322 (process ID) old priority 5, new priority 10

Rather than trying to explain the ancient problem (20 years old?),
just kill this text.

Signed-off-by: Michael Kerrisk <mtk.man-pages@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>