blk-mq: remove the error_count from struct request

The last two users were floppy.c and ataflop.c respectively, it was
verified that no other drivers makes use of this, so let's remove it.

Suggested-by: Linus Torvalds <[e-mail]>
Cc: Minh Yuan <[e-mail]>
Cc: Denis Efremov <[e-mail]>,
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Signed-off-by: Willy Tarreau <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/blk-mq.h

···
 		struct rb_node rb_node;	/* sort/lookup */
 		struct bio_vec special_vec;
 		void *completion_data;
-		int error_count; /* for legacy drivers, don't use */
 	};