Disable scraping the 'global' scope by default.

At Kumina, we use an ancient version of Dovecot on some of our systems
that doesn't support this scope yet.

The default set of scopes to be scraped was changed in #5.

dovecot_exporter.go

@@ -139,7 +139,7 @@ func main() {
 		listenAddress = app.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9166").String()
 		metricsPath   = app.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").String()
 		socketPath    = app.Flag("dovecot.socket-path", "Path under which to expose metrics.").Default("/var/run/dovecot/stats").String()
-		dovecotScopes = app.Flag("dovecot.scopes", "Stats scopes to query (comma separated)").Default("global,user").String()
+		dovecotScopes = app.Flag("dovecot.scopes", "Stats scopes to query (comma separated)").Default("user").String()
 	)
 	kingpin.MustParse(app.Parse(os.Args[1:]))