knotserver: simplify svg exception for now

Let's stick to just the file extension alone since we sometimes seem to get
text/plain as the MIME type.

knotserver/routes.go

···
 	mimeType := http.DetectContentType(contents)
 
 	// exception for svg
-	if strings.HasPrefix(mimeType, "text/xml") && filepath.Ext(treePath) == ".svg" {
+	if filepath.Ext(treePath) == ".svg" {
 		mimeType = "image/svg+xml"
 	}