Apply suggestions from code review

Co-authored-by: Florian Rathgeber <florian.rathgeber@gmail.com>

nbstripout/_utils.py

@@ -45,7 +45,7 @@ def get_size(item):
     if isinstance(item, str):
         return len(item)
     elif isinstance(item, list):
-        return functools.reduce(lambda a, b: a + get_size(b), item, 0)
+        return sum(get_size(elem) for elem in item)
     elif isinstance(item, dict):
         return get_size(list(item.values()))
     else:
@@ -121,12 +121,7 @@ def strip_output(nb, keep_output, keep_count, extra_keys='', max_size=0):
                     cell['outputs'] = []
                 else:
                     # otherwise remove outputs larger than max_size
-                    for idx, output in enumerate(cell['outputs']):
-                        if get_size(output) > max_size:
-                            cell['outputs'][idx] = None
-                        elif 'execution_count' in output and not keep_count:
-                            output['execution_count'] = None
-                    cell['outputs'] = [out for out in cell['outputs'] if out is not None]
+                    cell['outputs'] = [output for output in cell['outputs'] if get_size(output) <= max_size]
 
             # If keep_output_this_cell, but not keep_count, strip the counts
             # from the output.