floppy: cleanups: remove FLOPPY_SILENT_DCL_CLEAR undef

FLOPPY_SILENT_DCL_CLEAR is not defined anywhere and comes from pre-git
era. Just drop this undef. There is FD_SILENT_DCL_CLEAR which is really
used.

Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20210416083449.72700-6-efremov@linux.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>