[lint] Remove unnecessary lambdas

Lambdas of the form `lambda x: func(x)` are redundant.

discord/ext/commands/bot.py

@@ -137,7 +137,7 @@ async def _default_help_command(ctx, *commands : str):
         pages = await bot.formatter.format_help_for(ctx, command)
 
     if bot.pm_help is None:
-        characters = sum(map(lambda l: len(l), pages))
+        characters = sum(map(len, pages))
         # modify destination based on length of pages.
         if characters > 1000:
             destination = ctx.message.author