Drop redundant custom MarkDecorator __eq__ implementation

This is already covered by attrs.

Also, the custom implementation returns False when the types don't
match, but it's better to return `NotImplemented`. attrs does this.

src/_pytest/mark/structures.py

@@ -213,9 +213,6 @@ class MarkDecorator:
     def markname(self):
         return self.name  # for backward-compat (2.4.1 had this attr)
 
-    def __eq__(self, other):
-        return self.mark == other.mark if isinstance(other, MarkDecorator) else False
-
     def __repr__(self):
         return "<MarkDecorator {!r}>".format(self.mark)