Remove all SL4A related code (#939)

SL4A in Mobly has been deprecated for many years. Now it's time to finally remove all SL4A related code for good.

mobly/controllers/android_device.py

@@ -587,28 +587,6 @@ def add_device_info(self, name, info):
     """
     self._user_added_device_info.update({name: info})
 
-  @property
-  def sl4a(self):
-    """Attribute for direct access of sl4a client.
-
-    Not recommended. This is here for backward compatibility reasons.
-
-    Preferred: directly access `ad.services.sl4a`.
-    """
-    if self.services.has_service_by_name('sl4a'):
-      return self.services.sl4a
-
-  @property
-  def ed(self):
-    """Attribute for direct access of sl4a's event dispatcher.
-
-    Not recommended. This is here for backward compatibility reasons.
-
-    Preferred: directly access `ad.services.sl4a.ed`.
-    """
-    if self.services.has_service_by_name('sl4a'):
-      return self.services.sl4a.ed
-
   @property
   def debug_tag(self):
     """A string that represents a device object in debug info. Default value