dm-flakey: remove useless ERROR_READS check in flakey_end_io

If ERROR_READS is set, flakey_map returns DM_MAPIO_KILL for read
bios and flakey_end_io is never called, so there's no point in
checking it there. Also clean up an incorrect comment about when
read IOs are errored out.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>