Use UNWIND_PROTECT unconditionally

Rcpp has had support for 'UNWIND_PROTECT' for years, but behind a `#define` with an additional toggle to not define it.  In practice, it was always on.  As R 3.5.0 or later is now the baseline along with the C++11 as the minimally supported standard, we can remove the 'conditionalness' and just turn UNWIND_PROTECT on all the time.  The change is minimal, and passes tests locally.