Remove leftover ZEND_CAST code for (unset) cast.

Remove leftover ZEND_CAST code for (unset) cast.

Followup for d74d3922ce6f9ea07c1226b0cb2a94fc333f7a02

Attempting to require a file with (unset) casts results in an E_COMPILE_ERROR
that can't be caught or handled by set_exception_handler/set_error_handler.

Also remove the (bool) cast, because the ZEND_BOOL opcode handles that.

Remove inference that array -> object cast can throw.
It was added in 2a286ad5991 - I don't know how creating an stdClass would throw.
(numeric keys, references, etc. don't cause it to throw)

Closes GH-5042