Remove CustomPath's TextOutCustomPath method.

You can't really do anything useful with this in the form it currently
exists; among other problems, there's no way to reread whatever
information might be produced when the path is output.  Work is
underway to replace this with a more useful and more general system of
extensible nodes, but let's start by getting rid of this bit.

Extracted from a larger patch by KaiGai Kohei.