Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

models/user/email_address.go

@@ -147,11 +147,6 @@ func InsertEmailAddress(ctx context.Context, email *EmailAddress) (*EmailAddress
 	return email, nil
 }
 
-func UpdateEmailAddress(ctx context.Context, email *EmailAddress) error {
-	_, err := db.GetEngine(ctx).ID(email.ID).AllCols().Update(email)
-	return err
-}
-
 // ValidateEmail check if email is a valid & allowed address
 func ValidateEmail(email string) error {
 	if err := validateEmailBasic(email); err != nil {