Fix 0013181: Remove obsolete g_ldap_port from config_defaults_inc.php

$g_ldap_port is obsolete since 1.2.0a1, but the variable was not removed from
config_defaults_inc.php, resulting in a false warning in admin/check.php.

This problem was resolved in commit c207d0452631f6f5648e1813c807bce4873acc20
for 1.3.x but the change was never applied to 1.2.x.

This commit removes the obsolete variable, and aligns the comment for
$g_ldap_server to match the same in 1.3.x