http/httpproxy: remove comment on https proxy precedance

For golang/go#40909
For golang/go#54890

Change-Id: I1de1803f8fd00f54290404a8760d9f704ff766c3
Reviewed-on: https://go-review.googlesource.com/c/net/+/428795
Auto-Submit: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

http/httpproxy/proxy.go

@@ -81,8 +81,7 @@ type config struct {
 
 // FromEnvironment returns a Config instance populated from the
 // environment variables HTTP_PROXY, HTTPS_PROXY and NO_PROXY (or the
-// lowercase versions thereof). HTTPS_PROXY takes precedence over
-// HTTP_PROXY for https requests.
+// lowercase versions thereof).
 //
 // The environment values may be either a complete URL or a
 // "host[:port]", in which case the "http" scheme is assumed. An error