Removed setting override system due to confusing behaviour

- Was only used to disable registration when LDAP was enabled.
- Caused saved option not to show on settings page causing confusion.
- Extended setting logic where used to take ldap into account instead of
global override.
- Added warning on setting page to show registration enable setting is
not used while ldap is active.

For #1541

resources/lang/en/settings.php

@@ -56,6 +56,7 @@ return [
     'reg_enable_toggle' => 'Enable registration',
     'reg_enable_desc' => 'When registration is enabled user will be able to sign themselves up as an application user. Upon registration they are given a single, default user role.',
     'reg_default_role' => 'Default user role after registration',
+    'reg_enable_ldap_warning' => 'The option above is not used while LDAP authentication is active. User accounts for non-existing members will be auto-created if authentication, against the LDAP system in use, is successful.',
     'reg_email_confirmation' => 'Email Confirmation',
     'reg_email_confirmation_toggle' => 'Require email confirmation',
     'reg_confirm_email_desc' => 'If domain restriction is used then email confirmation will be required and this option will be ignored.',