Remove the argpar_parse() API

Remove the argpar_parse() API

Since the introduction of the iterator API, argpar_parse() is considered
a subset of what the iterator API can do, only adding a conditional (for
`fail_on_unknown_opt`) and creating an array of parsing items for the
user.

Therefore, as you can trivially implement an argpar_parse() equivalent
outside argpar, I see no interest in keeping it in the project.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I73857fef62de052457c9c7c4b0aab66187e81c27