build: partially replace gometalinter with golangci-lint (#16610)

we still use gometalinter for goconst since it doesn't 
report errors for duplicated in test files

pkg/api/login.go

@@ -164,7 +164,7 @@ func tryGetEncryptedCookie(ctx *m.ReqContext, cookieName string) (string, bool)
 		return "", false
 	}
 
-	decryptedError, err := util.Decrypt([]byte(decoded), setting.SecretKey)
+	decryptedError, err := util.Decrypt(decoded, setting.SecretKey)
 	return string(decryptedError), err == nil
 }