Remove unused deprecated functions

[barnabywalters]

There are some old, long-deprecated functions like https://github.com/microformats/php-mf2/blob/main/Mf2/Parser.php#L109 which haven’t been used in the php-mf2 codebase for years, and were only kept around for back compat. Let’s remove any which are unused in the next 0.x release (presumably 0.6)