Bluetooth: SDP: Don't clear l2cap chan on disconnect

- Remove memset on bt_l2cap_br_chan during SDP
  disconnect to prevent overwriting resources
  (e.g., rtx_work) still in use by L2CAP,
  which handles channel cleanup itself.
- Delete unused partial_resp_queue to clean up code.

Signed-off-by: ZhongYao Luo <LuoZhongYao@gmail.com>