leaudio: Move canceling background connect to OnGattDisconnect

This needs to be done as canceling background disconnect removes gatt
client from the `app_hold_link`, and could lead to situation that
LeAudio device is disconnected when there is no other GATT services
connected.

Bug: 297419998
Test: atest bluetooth_le_audio_client_test
Tag: #feature
Change-Id: I374e7691d3b796d24553c031bb1f26d5bb7f3a51