lib/download: remove download retries
Download retries are flawed because if some data is downloaded and later download is started again the new data is appended to old data. This means that result is invalid even if new download was successful.
One option would be to truncate output for retry but output can be not only file but effectively any buffer. I am not aware of easy way to truncate FILE. We can truncate real file but it seems that there is no way to truncate memory objects such as created using fopencookie.
It is also questionable if retries were even usable for anything. Curl on its own tries to reestablish connection and letting it on Curl is in general even cleaner than doing some hacks.