Do not skip e-mail validation if $g_login_method != LDAP

Prior to this, function email_is_valid() always returned true when
$g_use_ldap_email = ON, even if $g_login_method != LDAP.

Even though this is not an usual case (when not using LDAP, the other
LDAP-related configuration options should be left to their default
values), this causes the actual validation to be skipped which could
lead to email sending errors.

Fixes 0014632