fetcher/curl: Stop using CURLOPT_LOW_SPEED_TIME/_LIMIT

They don't play nicely currently with HTTP2 where we may
have lots of requests queued.

https://github.com/ostreedev/ostree/issues/878#issuecomment-347228854

In practice anyways I think issues here are better solved on a higher level -
e.g. apps today can use an overall timeout on pulls and if they exceed the limit
set the cancellable.

Closes: #1349
Approved by: jlebon

tests/test-pull-mirrorlist.sh

@@ -60,7 +60,7 @@ cat > ${test_tmpdir}/ostree-srv/mirrorlist <<EOF
 
 # comment with empty lines around
 
-http://example.com/nonexistent
+$(cat ${test_tmpdir}/content_mirror1-address)/ostree/non-existent-repo
 
 $(cat ${test_tmpdir}/content_mirror1-address)/ostree/gnomerepo
 $(cat ${test_tmpdir}/content_mirror2-address)/ostree/gnomerepo