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.
7 jobs for download-no-retry in 5 minutes and 40 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Build
passed #378598
build-alpine

00:01:21

passed #378596
build-debian-stable

00:01:15

passed #378597
build-debian-unstable

00:01:15

 
  Test
failed #378599
check

00:02:13

passed #378601
lint-c

00:00:56

passed #378602
lint-lua

00:00:58

passed #378600
valgrind-memcheck

00:04:19

 
Name Stage Failure
failed
check Test
make[1]: *** [Makefile:2018: check-TESTS] Error 2
make[1]: Leaving directory '/builds/turris/updater/updater'
make: *** [Makefile:2309: check-am] Error 2
Uploading artifacts...
test-suite*.log: found 1 matching files
tests/*/*.log: found 53 matching files

tests/*/*.trs: found 53 matching files

Uploading artifacts to coordinator... ok
id=378599 responseStatus=201 Created token=kUzfJshy
ERROR: Job failed: exit code 1