Documentation/block: drop the request.rst file

Documentation/block/request.rst is outdated and should be removed.
Also delete its entry in the block/index.rst file.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20230507182606.12647-1-rdunlap@infradead.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>