[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.
this does not work on PHP 5.3 when passing a callback like ``array($object, $class)``. And the Process class accepts any callable, not only closures.