bluetooth: Remove callback from BluetoothGattConnection::Disconnect

In the process of implementing implement
BluetoothDevice::CreateGattConnection for Android and Mac platforms
we found a pattern mis match, where previously callbacks upon
actions such as disconnect were planned. However, Android and
CoreBluetooth APIs only send events with no contract for a 1:1
pairing with calls such as disconnecting.

Simplify our API by removing the BluetoothGattConnection::Disconnect
callback, as we don't need it.

Part of a 4 patch series:
https://crrev.com/1287753002 Remove Disconnect callback <<<
https://crrev.com/1284073002 Add adapter_ to BluetoothDevice
https://crrev.com/1292263002 BluetoothDevice::CreateGattConnection Impl
https://crrev.com/1256313002 Android Impl & tests.

BUG=520304

Review URL: https://codereview.chromium.org/1287753002

Cr-Commit-Position: refs/heads/master@{#349233}
diff --git a/device/bluetooth/bluetooth_gatt_connection.h b/device/bluetooth/bluetooth_gatt_connection.h
index 81e1c8e..1f90636 100644
--- a/device/bluetooth/bluetooth_gatt_connection.h
+++ b/device/bluetooth/bluetooth_gatt_connection.h
@@ -34,10 +34,9 @@
   // Returns true if this connection is open.
   virtual bool IsConnected() = 0;
 
-  // Disconnects this GATT connection and calls |callback| upon completion.
-  // After a successful invocation, the device may still remain connected due to
-  // other GATT connections.
-  virtual void Disconnect(const base::Closure& callback) = 0;
+  // Disconnects this GATT connection. The device may still remain connected due
+  // to other GATT connections.
+  virtual void Disconnect() = 0;
 
  protected:
   BluetoothGattConnection();