Pix icon renderable does not implement templatable

As per the docs here: https://docs.moodle.org/dev/Templates

The pix icon renderable does not implement templatable (export_for_template). Also the "helper" for pix does not allow classes, and bypasses the renderers so it cannot be modified by php in a theme.

A typical thing to do in themes is override pix_icon with a renderer than uses font awesome, this is not possible with the pix_helper so it should be removed.

We should fix this before the 2.9 release so the template system is in good shape when plugins start to use it.