bindings file: avoid definition line comments

[mara004]

Since #192 / #196, we have a reference bindings file in the repository.
However, each time pdfium headers change, definition lines may shift (as seen in 0c1a20d).
We don't want this to blow up the repository's history unnecessarily.
See if we can make ctypesgen skip these needless line comments.

[mara004]

Hmm, I have a patch adding an option for this to ctypesgen, but using the option in pypdfium2 would mean to break compatibility with mainline ctypesgen...
Apart from that, we need to revisit how our ctypesgen fork is managed, for I'm not able to isolate that latest patch into an independent branch as it interferes with both the argparse and strings patches.