Revert "use a finalizer to close the h2quic.RoundTripper"

This reverts commit 65cea185bd.

The finalizer may run even before the h2quic.RoundTripper
variable falls out of scope. This can result in closing
the session before a HTTP transfer has completed.

h2quic/roundtrip.go

@@ -11,8 +11,6 @@ import (
 
 	quic "github.com/lucas-clemente/quic-go"
 
-	"runtime"
-
 	"golang.org/x/net/lex/httplex"
 )
 
@@ -93,7 +91,6 @@ func (r *RoundTripper) getClient(hostname string) http.RoundTripper {
 	defer r.mutex.Unlock()
 
 	if r.clients == nil {
-		runtime.SetFinalizer(r, finalizer)
 		r.clients = make(map[string]roundTripCloser)
 	}
 
@@ -105,10 +102,6 @@ func (r *RoundTripper) getClient(hostname string) http.RoundTripper {
 	return client
 }
 
-func finalizer(r *RoundTripper) {
-	_ = r.Close()
-}
-
 // Close closes the QUIC connections that this RoundTripper has used
 func (r *RoundTripper) Close() error {
 	r.mutex.Lock()