Remove unintuitive behaviour.

Previously, if you had `{@mention} {prefix}{command}`, `require_mention` was set to `True`, and `prefixes` were set to anything, then the command framework would look for a command named `{prefix}{command}`. However, this specific grouping of settings would not be chosen unless the bot creator wanted the command framework to look for `{command}`.

tl;dr settings weren't being respected

disco/bot/bot.py

@@ -351,7 +351,7 @@ class Bot(LoggingClass):
         # should be easy to avoid. An example of the unexpected results
         # that may occur would be if one prefix was `!` and one was `!a`.
         for prefix in prefixes:
-            if prefix and content.startswith(prefix) and not require_mention:  # don't cut off commands with mentions.
+            if prefix and content.startswith(prefix):
                 content = content[len(prefix):]
                 break
         else: