Remove obsolete conditions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

In `composer.json`, Twig version is set to 1.12 as a minimum, so all conditions about earlier Twig versions can be safely removed.