Trac #30281: remove __reduce__ method from Mutability

The class `Mutability` used to be a mixin class for mutable objects.
However, not every class inheriting from it uses pickling and even then
it must be overloaded manually in most cases anyway.

We suggest to remove that method from `Mutability`.

URL: https://trac.sagemath.org/30281
Reported by: gh-mjungmath
Ticket author(s): Michael Jung
Reviewer(s): Travis Scrimshaw, Matthias Koeppe

src/sage/structure/mutability.pyx

@@ -69,9 +69,6 @@ cdef class Mutability:
     def is_mutable(self):
         return not self._is_immutable
 
-    def __reduce__(self):
-        return Mutability, (self._is_immutable, )
-
 ##########################################################################
 ## Method decorators for mutating methods resp. methods that assume immutability