doc:locking: remove info about old behavior of locktorture

It is not useful to know what was the default at some point in the
past: remove the information.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Link: https://lore.kernel.org/r/20200426211429.29133-1-federico.vaga@vaga.pv.it


Signed-off-by: Jonathan Corbet <corbet@lwn.net>