remove xml and yaml output for api.php

[Klap-in]

in api.php a xml and yaml output is offered as well, using some included libraries (that aren't updated).

• xml is discouraged at https://www.dokuwiki.org/plugin:repository:api
• yaml, xml and json no use known.
• yaml, xml are not maintained libraries.
• the Extension Manager uses the php format
• json is more common format

conclusion: remove yaml and xml support?

        case 'xml':
            header('Content-Type: application/xml; charset=utf-8');
            require(__DIR__ . '/classes/A2Xml.php');
            $xml = xml_encode((object) $extensions, "hash");
            echo $xml;
            break;
        case 'yaml':
            header('Content-Type: text/yaml');
            require(__DIR__ . '/classes/Spyc.php');
            echo Spyc::YAMLDump($extensions, false, 0);
            break;

[splitbrain]

The extension manager now uses JSON. I'm fine with dropping xml and yaml.