throtl: don't specify sector_size for scsi_debug target

Currently, `scsi_debug` only supported a maximum `sector_size` of 4096
bytes. However, on systems with a `PAGE_SIZE` greater than 4KB, some
`throtl` tests (002, 003, and 007) attempt to load `scsi_debug`
with `sector_size=$PAGE_SIZE`.

This mismatch causes `scsi_debug` module loading to fail, resulting in
test failures such as:

> throtl/003 (sdebug) (bps limit over IO split)                [failed]
>     runtime  0.147s  ...  0.146s
>     --- tests/throtl/003.out    2025-04-04 04:36:43.175999880 +0800
>     +++ /root/blktests/results/nodev_sdebug/throtl/003.out.bad  2025-11-16 09:26:07.287964459 +0800
>     @@ -1,4 +1,3 @@
>      Running throtl/003
>     -1
>     -1
>     -Test complete
>     +modprobe: ERROR: could not insert 'scsi_debug': Invalid argument
>     +Loading scsi_debug dev_size_mb=1024 sector_size=65536 failed

After discussion[1] in the community, simply remove this parameter to leave
it as the default sector size.

[1] https://lore.kernel.org/linux-block/3407c6bc-9832-41d5-81c7-7216dd81f5e2@fnnas.com/

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>

Author: zhijianli88

tests/throtl/rc

@@ -76,7 +76,6 @@ _configure_throtl_blkdev() {
 		;;
 	sdebug)
 		args=(dev_size_mb=1024)
-		((sector_size)) && args+=(sector_size="${sector_size}")
 		if _configure_scsi_debug "${args[@]}"; then
 			THROTL_DEV=${SCSI_DEBUG_DEVICES[0]}
 			return