Only view milestones from current repo (#18414)

The endpoint /{username}/{reponame}/milestone/{id} is not currently restricted to
the repo. This PR restricts the milestones to those within the repo.

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/issue_milestone.go

@@ -134,22 +134,6 @@ func GetMilestoneByRepoIDANDName(repoID int64, name string) (*Milestone, error)
 	return &mile, nil
 }
 
-// GetMilestoneByID returns the milestone via id .
-func GetMilestoneByID(id int64) (*Milestone, error) {
-	return getMilestoneByID(db.GetEngine(db.DefaultContext), id)
-}
-
-func getMilestoneByID(e db.Engine, id int64) (*Milestone, error) {
-	var m Milestone
-	has, err := e.ID(id).Get(&m)
-	if err != nil {
-		return nil, err
-	} else if !has {
-		return nil, ErrMilestoneNotExist{ID: id, RepoID: 0}
-	}
-	return &m, nil
-}
-
 // UpdateMilestone updates information of given milestone.
 func UpdateMilestone(m *Milestone, oldIsClosed bool) error {
 	ctx, committer, err := db.TxContext()