avoid fatal error when using form_enctype

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12824, #12825
| License       | MIT
| Doc PR        | 

There seems to have been changes that crossed each other and a missing test... This is reverting #8731 as meanwhile, the line to output the deprecation was uncommented, breaking all twig templates that use form_enctype with a fatal php error.