Cleanup: $deadnotify assigned but never referenced in thold_check_threshold

[somethingwithproof]

Summary

In thold_functions.php (line 2261):

$deadnotify = (read_config_option('alert_deadnotify') == 'on');

This flag is fetched from config on every threshold evaluation but never used anywhere in the function. It is used in the separate dead-device notification path (line ~6879+), but is vestigial here. This contributes a pointless DB read per threshold per cycle.

Fix

Remove the $deadnotify assignment from thold_check_threshold().