update go.mod

Author: JuwanXu

server/go.mod

@@ -6,7 +6,6 @@ require (
 	github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751
 	github.com/casbin/casbin v1.9.1
 	github.com/casbin/gorm-adapter v1.0.0
-	github.com/dchest/captcha v0.0.0-20170622155422-6a29415a8364
 	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/fastly/go-utils v0.0.0-20180712184237-d95a45783239 // indirect
 	github.com/fsnotify/fsnotify v1.4.9