Revert "make modules/structs as a spereate go mod (#7127)" (#7175)

This reverts commit 835b53fc25.

modules/structs/attachment.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 package structs // import "code.gitea.io/gitea/modules/structs"
-
 import (
 	"time"
 )

modules/structs/go.mod

@@ -1,3 +0,0 @@
-module code.gitea.io/gitea/modules/structs
-
-go 1.12