cve-update-db-native: clean up proxy handling

urllib handles adding proxy handlers if the proxies are set in the environment,
so call bb.utils.export_proxies() to do that and remove the manual setup.

(From OE-Core rev: 6b730046)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>