[lint] Remove unnecessary lambdas

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

discord/calls.py

@@ -110,7 +110,7 @@ class GroupCall:
         lookup = {u.id: u for u in self.call.channel.recipients}
         me = self.call.channel.me
         lookup[me.id] = me
-        self.ringing = list(filter(None, map(lambda i: lookup.get(i), kwargs.get('ringing', []))))
+        self.ringing = list(filter(None, map(lookup.get, kwargs.get('ringing', []))))
 
     def _update_voice_state(self, data):
         user_id = int(data['user_id'])

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

discord/message.py

@@ -366,7 +366,7 @@ class Message:
     def channel_mentions(self):
         if self.guild is None:
             return []
-        it = filter(None, map(lambda m: self.guild.get_channel(m), self.raw_channel_mentions))
+        it = filter(None, map(self.guild.get_channel, self.raw_channel_mentions))
         return utils._unique(it)
 
     @utils.cached_slot_property('_cs_clean_content')