docs: block: biodoc.rst: Drop the obsolete and incorrect content

Since commit 7eaceac ("block: remove per-queue plugging"), kernel
has removed blk_run_address_space(), blk_unplug() and sync_buffer(),
and moved to on-stack plugging. The document has been obsolete for
years.
Given that there is no obvious counterparts in the new mechinism to
replace old APIs, this patch drops the content directly.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Link: https://lore.kernel.org/r/20220207074931.20067-1-song.bao.hua@hisilicon.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/biodoc.rst

@@ -965,11 +965,7 @@ till it fills up with a few more requests, before starting to service
 the requests. This provides an opportunity to merge/sort the requests before
 passing them down to the device. There are various conditions when the queue is
 unplugged (to open up the flow again), either through a scheduled task or
-could be on demand. For example wait_on_buffer sets the unplugging going
-through sync_buffer() running blk_run_address_space(mapping). Or the caller
-can do it explicity through blk_unplug(bdev). So in the read case,
-the queue gets explicitly unplugged as part of waiting for completion on that
-buffer.
+could be on demand.
 
 Aside:
   This is kind of controversial territory, as it's not clear if plugging is