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 <torvalds@linuxfoundation.org>
Cc: Minh Yuan <yuanmingbuaa@gmail.com>
Cc: Denis Efremov <efremov@linux.com>,
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>