[commands] Remove message being required from Role/Member converters.

This allows for easier "mock" context objects, for those who use
converters as utility functions outside of commands, and it's more
straightforward with the rest of the file.

discord/ext/commands/converter.py

@@ -98,10 +98,9 @@ class MemberConverter(IDConverter):
     """
 
     async def convert(self, ctx, argument):
-        message = ctx.message
         bot = ctx.bot
         match = self._get_id_match(argument) or re.match(r'<@!?([0-9]+)>$', argument)
-        guild = message.guild
+        guild = ctx.guild
         result = None
         if match is None:
             # not a mention...
@@ -316,7 +315,7 @@ class RoleConverter(IDConverter):
     3. Lookup by name
     """
     async def convert(self, ctx, argument):
-        guild = ctx.message.guild
+        guild = ctx.guild
         if not guild:
             raise NoPrivateMessage()