Remove usage of HAS_UNWIND_PROTECT

[DavisVaughan]

cpp11 is bumping to require R >=4.0.0 in the next release r-lib/cpp11#411

That means that R_UnwindProtect() is always available, and the macro HAS_UNWIND_PROTECT is no longer needed. I'm going to keep it around for a release or two, but if you could remove your reliance on it, then that'd be great!

systemfonts/src/utils.h

Line 11 in 02b5670

#ifdef HAS_UNWIND_PROTECT