block_dump: remove comments in docs

Now block_dump feature is gone, remove all comments in docs.
Signed-off-by: Nzhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: NJan Kara <jack@suse.cz>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210313030146.2882027-4-yi.zhang@huawei.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

Documentation/admin-guide/laptops/laptop-mode.rst

@@ -101,17 +101,6 @@ this results in concentration of disk activity in a small time interval which
 occurs only once every 10 minutes, or whenever the disk is forced to spin up by
 a cache miss. The disk can then be spun down in the periods of inactivity.
 
-If you want to find out which process caused the disk to spin up, you can
-gather information by setting the flag /proc/sys/vm/block_dump. When this flag
-is set, Linux reports all disk read and write operations that take place, and
-all block dirtyings done to files. This makes it possible to debug why a disk
-needs to spin up, and to increase battery life even more. The output of
-block_dump is written to the kernel output, and it can be retrieved using
-"dmesg". When you use block_dump and your kernel logging level also includes
-kernel debugging messages, you probably want to turn off klogd, otherwise
-the output of block_dump will be logged, causing disk activity that is not
-normally there.
-
 
 Configuration
 -------------

Documentation/admin-guide/sysctl/vm.rst

@@ -25,7 +25,6 @@ files can be found in mm/swap.c.
 Currently, these files are in /proc/sys/vm:
 
 - admin_reserve_kbytes
-- block_dump
 - compact_memory
 - compaction_proactiveness
 - compact_unevictable_allowed
@@ -106,13 +105,6 @@ On x86_64 this is about 128MB.
 Changing this takes effect whenever an application requests memory.
 
 
-block_dump
-==========
-
-block_dump enables block I/O debugging when set to a nonzero value. More
-information on block I/O debugging is in Documentation/admin-guide/laptops/laptop-mode.rst.
-
-
 compact_memory
 ==============