remove irrelevant docs in optimizer_step (#4964)

Co-authored-by: chaton <thomas@grid.ai>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

pytorch_lightning/core/lightning.py

@@ -1232,10 +1232,6 @@ def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_idx,
                     optimizer.step(closure=optimizer_closure)
                     optimizer.zero_grad()
 
-        Note:
-            If you also override the :meth:`~pytorch_lightning.core.hooks.ModelHooks.on_before_zero_grad`
-            model hook don't forget to add the call to it before ``optimizer.zero_grad()`` yourself.
-
         """
         if not isinstance(optimizer, LightningOptimizer):
             # wraps into LightingOptimizer only for running step