md: drop queue limitation for RAID1 and RAID10

As suggested by Neil Brown[1], this limitation seems to be
deprecated.

With plugging in use, writes are processed behind the raid thread
and conf->pending_count is not increased. This limitation occurs only
if caller doesn't use plugs.

It can be avoided and often it is (with plugging). There are no reports
that queue is growing to enormous size so remove queue limitation for
non-plugged IOs too.

[1] https://lore.kernel.org/linux-raid/162496301481.7211.18031090130574610495@noble.neil.brown.name

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Song Liu <song@kernel.org>