Remove deprecated code

[swissspidy]

As a follow-up to #876, where AMP_WP_Utils was deprecated, I think it is now time to fully remove that code.

A quick search on https://wpdirectory.net/ revealed exactly 0 plugins and themes that use any of these utils. There are only plugins (e.g. AMP for WP) that also have this class, but that's because they include this plugin here as a Composer dependency.

We could remove the code in the next bigger release, e.g. 1.3.0 or 2.0.0 the latest.

Similarly, we could remove other code that was deprecated in < 1.0, e.g. amp_load_classes()