tests: Bluetooth: tester: Do not memset the streams on ACL disconnection

The streams can be deallocated once bt_bap_stream_ops.released callback
is called. Otherwise it may cause memory violation in the stack.

Fixes: ASCS/SR/ACP/BV-31-C
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>