block.h: remove outdated comment

block.h: remove outdated comment

The comment "disk I/O throttling" doesn't make any sense at all
any more. It was added in commit 0563e191516 to describe
bdrv_io_limits_enable()/disable(), which were removed in commit
97148076, so the comment is just a forgotten leftover.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220131125615.74612-1-eesposit@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>