Remove property from columns attribute on Transformable protocol

When a property is declared on a protocol. When used with an
isinstance check this results in the property being called, which can
have unintended side-effects and impact on performance.

merlin/core/protocols.py

@@ -91,7 +91,6 @@ class Transformable(DictLike, Protocol):
     base to do without them.
     """
 
-    @property
     def columns(self):
         ...