remove qs.slave_okay() that is deprecated since pymongo3

mongoengine/queryset/base.py

@@ -60,7 +60,6 @@ class BaseQuerySet(object):
         self._ordering = None
         self._snapshot = False
         self._timeout = True
-        self._slave_okay = False
         self._read_preference = None
         self._iter = False
         self._scalar = []
@@ -775,7 +774,6 @@ class BaseQuerySet(object):
             "_ordering",
             "_snapshot",
             "_timeout",
-            "_slave_okay",
             "_read_preference",
             "_iter",
             "_scalar",
@@ -1172,20 +1170,6 @@ class BaseQuerySet(object):
         queryset._timeout = enabled
         return queryset
 
-    # DEPRECATED. Has no more impact on PyMongo 3+
-    def slave_okay(self, enabled):
-        """Enable or disable the slave_okay when querying.
-
-        :param enabled: whether or not the slave_okay is enabled
-
-        .. deprecated:: Ignored with PyMongo 3+
-        """
-        msg = "slave_okay is deprecated as it has no impact when using PyMongo 3+."
-        warnings.warn(msg, DeprecationWarning)
-        queryset = self.clone()
-        queryset._slave_okay = enabled
-        return queryset
-
     def read_preference(self, read_preference):
         """Change the read_preference when querying.