fix(netxlite): do not mutate outgoing requests (#508)

I have recently seen a data race related our way of
mutating the outgoing request to set the host header.

Unfortunately, I've lost track of the race output,
because I rebooted my Linux box before saving it.

Though, after inspecting why and and where we're mutating
outgoing requets, I've found that:

1. we add the host header when logging to have it logged,
which is not a big deal since we already emit the URL
rather than just the URL path when logging a request, and
so we can safely zap this piece of code;

2. as a result, in measurements we may omit the host header
but again this is pretty much obvious from the URL itself
and so it should not be very important (nonetheless,
avoid surprises and keep the existing behavior);

3. when the User-Agent header is not set, we default to
a `miniooni/0.1.0-dev` user agent, which is probably not
very useful anyway, so we can actually remove it.

Part of https://github.com/ooni/probe/issues/1733 (this diff
has been extracted from https://github.com/ooni/probe-cli/pull/506).

internal/engine/legacy/netx/oldhttptransport/httptransport.go

@@ -22,11 +22,6 @@ func New(roundTripper http.RoundTripper) *Transport {
 // RoundTrip executes a single HTTP transaction, returning
 // a Response for the provided Request.
 func (t *Transport) RoundTrip(req *http.Request) (resp *http.Response, err error) {
-	// Make sure we're not sending Go's default User-Agent
-	// if the user has configured no user agent
-	if req.Header.Get("User-Agent") == "" {
-		req.Header["User-Agent"] = nil
-	}
 	return t.roundTripper.RoundTrip(req)
 }