Remove unused DeviceRow class (#6800)

changelog.d/6800.bugfix

+Fix race in federation sender worker that delayed sending of device updates.

synapse/federation/send_queue.py

@@ -454,28 +454,9 @@ class EduRow(BaseFederationRow, namedtuple("EduRow", ("edu",))):  # Edu
         buff.edus.setdefault(self.edu.destination, []).append(self.edu)
 
 
-class DeviceRow(BaseFederationRow, namedtuple("DeviceRow", ("destination",))):  # str
-    """Streams the fact that either a) there is pending to device messages for
-    users on the remote, or b) a local users device has changed and needs to
-    be sent to the remote.
-    """
-
-    TypeId = "d"
-
-    @staticmethod
-    def from_data(data):
-        return DeviceRow(destination=data["destination"])
-
-    def to_data(self):
-        return {"destination": self.destination}
-
-    def add_to_buffer(self, buff):
-        buff.device_destinations.add(self.destination)
-
-
 TypeToRow = {
     Row.TypeId: Row
-    for Row in (PresenceRow, PresenceDestinationsRow, KeyedEduRow, EduRow, DeviceRow)
+    for Row in (PresenceRow, PresenceDestinationsRow, KeyedEduRow, EduRow,)
 }