dont call downcase() on undef variable

This got probably introduced during some debugging. The variable is
always undef. This works on Puppet 4 and 5 because downcase() works with
undef by accident. This got fixed in the latest puppet 6. It now results
in a compile error.

manifests/extension.pp

@@ -110,7 +110,7 @@
     $_settings.each |$settings_name, $settings_hash| {
       $so_name = $multifile_settings ? {
         true  => downcase($settings_name),
-        false => pick(downcase($so_name), downcase($name), downcase($settings_name)),
+        false => pick(downcase($name), downcase($settings_name)),
       }
 
       php::extension::config { $settings_name: