Remove write type from GATT descriptor writes (3/4)

According to the Bluetooth Core specification v4.2, Vol 3, Part G,
section 4.12.3: "The Attribute Protocol WRITE REQUEST is used
for this sub-procedure".

Change-Id: I3bb34b52234b5c21a5ba3ec52e0c6de901c11fbe

include/hardware/bt_gatt_client.h

@@ -315,8 +315,7 @@ typedef struct {
 
     /** Write a remote descriptor for a given characteristic */
     bt_status_t (*write_descriptor)( int conn_id, uint16_t handle,
-                    int write_type, int auth_req,
-                    vector<uint8_t> value);
+                    int auth_req, vector<uint8_t> value);
 
     /** Execute a prepared write operation */
     bt_status_t (*execute_write)(int conn_id, int execute);