baseset: drop custom __add__ method

This add method is enforcing non-laziness, disabling multiple optimisations.

Benchmarks do not spot any significant differences but real usecase may. This
will also be important for further improvements to addset later in this series.

mercurial/revset.py

@@ -2388,14 +2388,6 b' class baseset(abstractsmartset):'
         This is part of the mandatory API for smartset."""
         return baseset([y for y in self if y in other])
 
-    def __add__(self, other):
-        """Returns a new object with the union of the two collections.
-
-        This is part of the mandatory API for smartset."""
-        s = self.set()
-        l = [r for r in other if r not in s]
-        return baseset(list(self) + l)
-
     def isascending(self):
         """Returns True if the collection is ascending order, False if not.