[commands] Remove Command.no_pm for commands.guild_only check.

This is a breaking change. The original purpose of no_pm has been
mainly a legacy aspect. They came from a time before checks were a
generalised concept and were never removed. A check is the proper way
to do its behaviour.

discord/ext/commands/errors.py

@@ -90,16 +90,16 @@ class BadArgument(UserInputError):
     """
     pass
 
-class NoPrivateMessage(CommandError):
+class CheckFailure(CommandError):
+    """Exception raised when the predicates in :attr:`Command.checks` have failed."""
+    pass
+
+class NoPrivateMessage(CheckFailure):
     """Exception raised when an operation does not work in private message
     contexts.
     """
     pass
 
-class CheckFailure(CommandError):
-    """Exception raised when the predicates in :attr:`Command.checks` have failed."""
-    pass
-
 class DisabledCommand(CommandError):
     """Exception raised when the command being invoked is disabled."""
     pass