[Form] Fix call to removed method (BC broken in 2.3)

ResolvedFormType#getTypeExtensions() was calling method
AbstractType#getExtensions() which has been removed in
commit b3081e8

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

src/Symfony/Component/Form/ResolvedFormType.php

@@ -93,11 +93,6 @@ public function getInnerType()
      */
     public function getTypeExtensions()
     {
-        // BC
-        if ($this->innerType instanceof AbstractType) {
-            return $this->innerType->getExtensions();
-        }
-
         return $this->typeExtensions;
     }