bluetooth: host: att: Remove att_sent function

`att_sent` is removed as it does not provide any value. All checks are
already performed in `att_on_sent_cb`, and keeping it only increases
readability complexity.

`att_sent` is removed as doesn't give any value. All checks are done
already in `att_on_sent_cb`. It just increases readness complexity.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>