raid5: remove STRIPE_OPS_REQ_PENDING

This stripe state is not used anymore after commit 51acbcec
("md: remove CONFIG_MULTICORE_RAID456"), so remove the obsoleted
state.

gjiang@nb01257:~/md$ grep STRIPE_OPS_REQ_PENDING drivers/md/ -r
drivers/md/raid5.c:					  (1 << STRIPE_OPS_REQ_PENDING) |
drivers/md/raid5.h:	STRIPE_OPS_REQ_PENDING,

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>