MDL-19077 - Finish eliminating customcorners-specific code in core.

MDL-19077 - Finish eliminating customcorners-specific code in core.

The custom-corners-specific code now in theme/customcornser/renderers.php
and lib/deprecatedlib.php.

Also, $CFG->pixpath is now causing more problems than ever. If it is
giving your problems, please call $OUTPUT->initialise_deprecated_cfg_pixpath()
as a temporary fix. As you can imagine, we are thinking about a better
long-term fix, which is why that method as a silly, and easy to grep name.