Refactor DeleteInactiveUsers, fix bug and add tests (#30206)

1. check `IsActive` before calling `IsLastAdminUser`.
2. Fix some comments and error messages.
3. Don't `return err` if "removing file" fails in `DeleteUser`.
4. Remove incorrect `DeleteInactiveEmailAddresses`. Active users could
also have inactive emails, and inactive emails do not support
"olderThan"
5. Add tests

models/user/email_address.go

@@ -256,14 +256,6 @@ func IsEmailUsed(ctx context.Context, email string) (bool, error) {
 	return db.GetEngine(ctx).Where("lower_email=?", strings.ToLower(email)).Get(&EmailAddress{})
 }
 
-// DeleteInactiveEmailAddresses deletes inactive email addresses
-func DeleteInactiveEmailAddresses(ctx context.Context) error {
-	_, err := db.GetEngine(ctx).
-		Where("is_activated = ?", false).
-		Delete(new(EmailAddress))
-	return err
-}
-
 // ActivateEmail activates the email address to given user.
 func ActivateEmail(ctx context.Context, email *EmailAddress) error {
 	ctx, committer, err := db.TxContext(ctx)