zram: remove unused stats fields

zram: remove unused stats fields

We don't show num_reads and num_writes since we removed corresponding
sysfs nodes in 2017.  Block layer stats are exposed via
/sys/block/zramX/stat file.

However, we still increment those atomic vars and store them in zram
stats.  Remove leftovers.

Link: https://lkml.kernel.org/r/20221117141326.1105181-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>