[BUG]: Volt: dead closure prefix

The closure prefix generator cannot be used

Compiler::getUniquePrefix() has a branch that calls the prefix as a closure, but setUniquePrefix() takes a string in both projects, so the only way in is a subclass writing the protected property. cphalcon declares protected prefix (untyped) so that works; phalcon has since widened its property to match, but the setter still refuses a closure on both sides.

Decide whether the closure prefix is supported. If yes, widen setUniquePrefix() in both. If no, drop the branch from both.