deprecate(libraries): End support for `elgg_register_library` and `elgg_load_library`

Class autoloading is better in every way. It's trivial to use static methods
on autoloaded classes rather than `elgg_*_library` with global functions.

This is a "soft" deprecation (no warnings/notices when using the deprecated APIs)
because we're not updating core plugins just yet.
Merge pull request #7443 from ewinslow/deprecate-libraries

deprecate(libraries): End support for `elgg_register_library` and `elgg_load_library`