dm thin: remove stale 'trim' message documentation

The 'trim' message wasn't ever implemented.

Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/device-mapper/thin-provisioning.txt

···
 load a target that is bigger than before, then extra blocks will be
 provisioned as and when needed.
 
-If you wish to reduce the size of your thin device and potentially
-regain some space then send the 'trim' message to the pool.
-
 ii) Status
 
      <nr mapped sectors> <highest mapped sector>