remove workaround for GCC 4.1-4.3 [PR105606]

While looking into vec.h, I've noticed we still have a workaround for
GCC 4.1-4.3 bugs.
As we now use C++11 and thus need to be built by GCC 4.8 or later,
I think this is now never used.

2023-09-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105606
	* system.h (BROKEN_VALUE_INITIALIZATION): Don't define.
	* vec.h (vec_default_construct): Remove BROKEN_VALUE_INITIALIZATION
	workaround.
	* function.cc (assign_parm_find_data_types): Likewise.