Remove ability to override builtins for rson factory methods
Remove ability to override builtins for rson factory methods

The vendored 'rst2pdf.rson' module provides a number of factory methods.
Many of these factory methods provide the ability to override Python
builtins by passing arguments, e.g.:

  Tokenizer.factory(len=some_other_len_function)

We weren't using any of this functionality, which means it's unnecessary
complexity that should be removed. Do just that.

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

rst2pdf/rst2pdf,rst2pdf/rst2pdf