download: more simplifications

[djc]

Following up to

• Simplify downloads #4844

Interestingly, it looks like a number of the tests time out on my macOS machine when run as part of a full cargo test --features test (even on main):

test download::tests::reqwest::network_failure_does_not_delete_partial_file has been running for over 60 seconds
test download::tests::reqwest::read_basic_proxy_params has been running for over 60 seconds
test download::tests::reqwest::resume_partial_fails_if_server_ignores_range has been running for over 60 seconds
test download::tests::reqwest::socks_proxy_request has been running for over 60 seconds
test download::tests::reqwest::network_failure_does_not_delete_partial_file ... ok

although they work okay when running cargo test --features test download::tests.