wkhtmltopdf: do not check return code of wkhtmltopdf
This partially reverts dd792169 (wkhtmltopdf: Improve error reporting, 2025-12-05), because ERP5 test suite revealed some usages where wkhtmltopdf process exits with a non-zero return code, for example when converting an html file referencing images as relative paths. Converting html with such images try to open local files, that are blocked because we use --disable-local-file-access, but this apparently affects the return code of wkhtmltopdf.
This also fixes testWkhtmltopdfServer so that it actually uses wkhtmltopdf.