refactor: use only a single implementation for cache purging

This change moves cache clearing into the repo plugin, and standardizes on a single way of clearing the cache.

plugins/repo/src/triggers/pre-delete/pre-delete.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"flag"
+
+	"github.com/dokku/dokku/plugins/common"
+	"github.com/dokku/dokku/plugins/repo"
+)
+
+// destroys the buildpacks property for a given app container
+func main() {
+	flag.Parse()
+	appName := flag.Arg(0)
+
+	err := repo.PurgeCache(appName)
+	if err != nil {
+		common.LogFail(err.Error())
+	}
+}