docs: drivers: remove suggestion to assign driver_api in init function

The driver API should be assigned at the time the device is defined.
Assigning in the init function is redundant and prevents moving the
pointer to a immutable driver structure.

Signed-off-by: Peter Bigot <[email protected]>

doc/reference/drivers/index.rst

@@ -173,8 +173,7 @@ of these APIs, and populate an instance of subsystem_api structure:
   };
 
 The driver would then pass ``my_driver_api_funcs`` as the ``api`` argument to
-``DEVICE_AND_API_INIT()``, or manually assign it to ``device->driver_api``
-in the driver init function.
+``DEVICE_AND_API_INIT()``.
 
 .. note::