Yadis: drop custom replaceEntities() function and use html_entity_decode

Yadis' ParseHTML.php has a replaceEntities() function for replacing HTML
entities, with a comment that appears to explain its existence by stating
"Replace numeric entities because html_entity_decode doesn't do it for us".
This is breaking with PHP 5.5 because it uses the deprecated /e modifier
for preg_replace() - https://github.com/openid/php-openid/issues/108 .
I think this custom function is no longer needed at least with PHP 5. PHP 5
has had support for replacing numeric entities since 2003, and its entity
handling code has been refined quite a lot since then. replaceEntities() has
been there since 2006, and probably earlier. I guess at that time PHP 4
compatibility was still important so this was needed - I don't think PHP 4's
html_entity_decode() has ever had numeric entity support - but now PHP 5 is
a decade old and there's a separate PHP 4 branch of php-openid, I think we
can ditch replaceEntities() in the main branch!