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

Since commit 7eaceacc ("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>