Let's get rid of ugly statik (#2255)

* Get rid of ugly statik

go1.16 introduced the embed package, it's the more graceful solution for embedding file into binary.
https://golang.org/pkg/embed/

* remove statik totally

* split go and static files in assets

assets/frps/embed.go

@@ -0,0 +1,14 @@
+package frpc
+
+import (
+	"embed"
+
+	"github.com/fatedier/frp/assets"
+)
+
+//go:embed static/*
+var content embed.FS
+
+func init() {
+	assets.Register(content)
+}