While deleting 3rd party module "ForumNG" and its sub tables...
I got the following error "Fatal error: Maximum execution time of 30 seconds exceeded" while deleting the 5th table on the list (probably a big one)
adding:

@ini_set('max_execution_time', 120);

before line (574) on: /lib/xmldb/xmldb_table.php

if (!$result) { //Skip on error

  continue;

}

@ini_set('max_execution_time', 120); // might take long time

$name = trim($xmlkey['@']['NAME']);

$key = new xmldb_key($name);

$key->arr2xmldb_key($xmlkey);

$this->keys[] = $key;

Worked a round the issue. And I was able to delete the main table and all the sub tables.
btw, my php.ini is set to max_execution_time = 30

We might consider adding that extra php execution time at a better location inside the code.