Remove prom-active-alerts

Prom-active-alerts command is not used, not tested, and
currently broken. Let's remove it.

.gitignore

@@ -1,4 +1,3 @@
 cmd/kured/kured
-cmd/prom-active-alerts/prom-active-alerts
 vendor
 build

cmd/prom-active-alerts/main.go

@@ -1,23 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"log"
-	"os"
-	"regexp"
-
-	"github.com/weaveworks/kured/pkg/alerts"
-)
-
-func main() {
-	if len(os.Args) != 3 {
-		log.Fatalf("USAGE: %s <prometheusURL> <filterRegexp>", os.Args[0])
-	}
-
-	count, err := alerts.PrometheusCountActive(os.Args[1], regexp.MustCompile(os.Args[2]))
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	fmt.Println(count)
-}