[2.3] Remove possible call_user_func()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Merging this in 2.3 enhances performance a bit, but more importantly will ease future merges into 3.0.
the code accepts any callable when setting it, so this would break on PHP 5.3 too (btw, the setter defines the type as ``string`` instead of ``callable``, which you may want to fix at the same time)